Revert "Reland "iOS: Migrate FlutterEngine to ARC" (#55937)" (#55954)

There are still a couple closures where on engine shutdown, the last
live reference to FlutterEngine may be on a thread other than the
platform thread.

Specifically, the profiling data capture callback can result in the last
live reference to a FlutterEngine being on a profiling thread, resulting
in an assertion failure in the destructor of the PlatformViewsController
held by FlutterEngine, because PlatformViewsController holds a
WeakPtrFactory whose destructor asserts that it be on the platform
thread.

See:

https://github.com/flutter/engine/blob/ad9e4fef451a73427285826455db5fbdde502bd7/shell/platform/darwin/ios/framework/Source/FlutterEngine.mm#L511-L515

Backtrace of such a crash:
```
=================================================================
Main Thread Checker: UI API called on a background thread: -[UIView removeFromSuperview]
PID: 46919, TID: 333147, Thread name: FlutterEngine.166.profiler, Queue name: com.apple.root.default-qos.overcommit, QoS: 0
Backtrace:
4   libios_test_flutter.dylib           0x000000014b658aed -[FlutterTextInputPlugin dealloc] + 27
5   libobjc.A.dylib                     0x00007ff800055228 _ZN11objc_object17sidetable_releaseEbb + 204
6   libios_test_flutter.dylib           0x000000014b63df5a -[FlutterEngine .cxx_destruct] + 135
7   libobjc.A.dylib                     0x00007ff800035766 _ZL27object_cxxDestructFromClassP11objc_objectP10objc_class + 83
8   libobjc.A.dylib                     0x00007ff80004ddfc objc_destructInstance + 61
9   CoreFoundation                      0x00007ff8004a286b -[NSObject(NSObject) __dealloc_zombie] + 159
10  libios_test_flutter.dylib           0x000000014b635fea -[FlutterEngine dealloc] + 334
11  libobjc.A.dylib                     0x00007ff800055228 _ZN11objc_object17sidetable_releaseEbb + 204
12  libios_test_flutter.dylib           0x000000014b63eda9 _ZNSt3_fl10__function6__funcIZ30-[FlutterEngine startProfiler]E3$_0NS_9allocatorIS2_EEFN7flutter13ProfileSampleEvEEclEv + 69
13  libios_test_flutter.dylib           0x000000014bcd8eaa _ZNSt3_fl10__function6__funcIZNK7flutter16SamplingProfiler16SampleRepeatedlyEN3fml9TimeDeltaEE3$_0NS_9allocatorIS6_EEFvvEEclEv + 40
14  libios_test_flutter.dylib           0x000000014b868786 _ZN3fml15MessageLoopImpl10FlushTasksENS_9FlushTypeE + 156
15  libios_test_flutter.dylib           0x000000014b86ecca _ZN3fml17MessageLoopDarwin11OnTimerFireEP16__CFRunLoopTimerPS0_ + 26
16  CoreFoundation                      0x00007ff8003ea4a5 __CFRUNLOOP_IS_CALLING_OUT_TO_A_TIMER_CALLBACK_FUNCTION__ + 20
17  CoreFoundation                      0x00007ff8003ea032 __CFRunLoopDoTimer + 801
18  CoreFoundation                      0x00007ff8003e97b6 __CFRunLoopDoTimers + 243
19  CoreFoundation                      0x00007ff8003e4028 __CFRunLoopRun + 2108
20  CoreFoundation                      0x00007ff8003e3409 CFRunLoopRunSpecific + 557
21  libios_test_flutter.dylib           0x000000014b86ee07 _ZN3fml17MessageLoopDarwin3RunEv + 65
22  libios_test_flutter.dylib           0x000000014b8686a4 _ZN3fml15MessageLoopImpl5DoRunEv + 22
23  libios_test_flutter.dylib           0x000000014b86dce5 _ZNSt3_fl10__function6__funcIZN3fml6ThreadC1ERKNS_8functionIFvRKNS3_12ThreadConfigEEEES7_E3$_0NS_9allocatorISC_EEFvvEEclEv + 135
24  libios_test_flutter.dylib           0x000000014b86da87 _ZZN3fml12ThreadHandleC1EONSt3_fl8functionIFvvEEEEN3$_08__invokeEPv + 27
25  libsystem_pthread.dylib             0x000000010d6a818b _pthread_start + 99
26  libsystem_pthread.dylib             0x000000010d6a3ae3 thread_start + 15
2024-10-18 09:31:27.549111-0700 IosUnitTests[46919:333147] [reports] Main Thread Checker: UI API called on a background thread: -[UIView removeFromSuperview]
PID: 46919, TID: 333147, Thread name: FlutterEngine.166.profiler, Queue name: com.apple.root.default-qos.overcommit, QoS: 0
Backtrace:
4   libios_test_flutter.dylib           0x000000014b658aed -[FlutterTextInputPlugin dealloc] + 27
5   libobjc.A.dylib                     0x00007ff800055228 _ZN11objc_object17sidetable_releaseEbb + 204
6   libios_test_flutter.dylib           0x000000014b63df5a -[FlutterEngine .cxx_destruct] + 135
7   libobjc.A.dylib                     0x00007ff800035766 _ZL27object_cxxDestructFromClassP11objc_objectP10objc_class + 83
8   libobjc.A.dylib                     0x00007ff80004ddfc objc_destructInstance + 61
9   CoreFoundation                      0x00007ff8004a286b -[NSObject(NSObject) __dealloc_zombie] + 159
10  libios_test_flutter.dylib           0x000000014b635fea -[FlutterEngine dealloc] + 334
11  libobjc.A.dylib                     0x00007ff800055228 _ZN11objc_object17sidetable_releaseEbb + 204
12  libios_test_flutter.dylib           0x000000014b63eda9 _ZNSt3_fl10__function6__funcIZ30-[FlutterEngine startProfiler]E3$_0NS_9allocatorIS2_EEFN7flutter13ProfileSampleEvEEclEv + 69
13  libios_test_flutter.dylib           0x000000014bcd8eaa _ZNSt3_fl10__function6__funcIZNK7flutter16SamplingProfiler16SampleRepeatedlyEN3fml9TimeDeltaEE3$_0NS_9allocatorIS6_EEFvvEEclEv + 40
14  libios_test_flutter.dylib           0x000000014b868786 _ZN3fml15MessageLoopImpl10FlushTasksENS_9FlushTypeE + 156
15  libios_test_flutter.dylib           0x000000014b86ecca _ZN3fml17MessageLoopDarwin11OnTimerFireEP16__CFRunLoopTimerPS0_ + 26
16  CoreFoundation                      0x00007ff8003ea4a5 __CFRUNLOOP_IS_CALLING_OUT_TO_A_TIMER_CALLBACK_FUNCTION__ + 20
17  CoreFoundation                      0x00007ff8003ea032 __CFRunLoopDoTimer + 801
18  CoreFoundation                      0x00007ff8003e97b6 __CFRunLoopDoTimers + 243
19  CoreFoundation                      0x00007ff8003e4028 __CFRunLoopRun + 2108
20  CoreFoundation                      0x00007ff8003e3409 CFRunLoopRunSpecific + 557
21  libios_test_flutter.dylib           0x000000014b86ee07 _ZN3fml17MessageLoopDarwin3RunEv + 65
22  libios_test_flutter.dylib           0x000000014b8686a4 _ZN3fml15MessageLoopImpl5DoRunEv + 22
23  libios_test_flutter.dylib           0x000000014b86dce5 _ZNSt3_fl10__function6__funcIZN3fml6ThreadC1ERKNS_8functionIFvRKNS3_12ThreadConfigEEEES7_E3$_0NS_9allocatorISC_EEFvvEEclEv + 135
24  libios_test_flutter.dylib           0x000000014b86da87 _ZZN3fml12ThreadHandleC1EONSt3_fl8functionIFvvEEEEN3$_08__invokeEPv + 27
25  libsystem_pthread.dylib             0x000000010d6a818b _pthread_start + 99
26  libsystem_pthread.dylib             0x000000010d6a3ae3 thread_start + 15
IosUnitTests(47009,0x10ec76240) malloc: enabling scribbling to detect mods to free blocks
IosUnitTests(47009) MallocStackLogging: could not tag MSL-related memory as no_footprint, so those pages will be included in process footprint - (null)
IosUnitTests(47009) MallocStackLogging: stack logs being written to /private/tmp/stack-logs.47009.1057b6000.IosUnitTests.0o4HgN.index
IosUnitTests(47009) MallocStackLogging: recording malloc and VM allocation stacks to disk using standard recorder
IosUnitTests(47009) MallocStackLogging: process 46919 no longer exists, stack logs deleted from /tmp/stack-logs.46919.10d1fc000.IosUnitTests.kFionm.index
```

This reverts commit 02fc8a455d0c4b70a70d36e7f4329f5ad706596d.

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