Revert "iOS: Migrate FlutterEngine to ARC (#55590)" (#55699)

Causing flakes on the _Mac mac_unopt (Cocoon)_ shard.

This reverts commit 449df252a2f13a10aced5ea1d7000cf4e276d71a.

Issue: https://github.com/flutter/flutter/issues/156177

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
https://dart.googlesource.com/external/github.com/flutter/engine/+/2484ae11b5eed2632658081f6d877911517dde2c

DEPS

diff --git a/DEPS b/DEPS
index d6dc83b..d07c17b 100644
--- a/DEPS
+++ b/DEPS
@@ -280,7 +280,7 @@
 deps = {
   'engine/src': 'https://github.com/flutter/buildroot.git' + '@' + 'f85c3be4bf808add6ba867b8ff7943fd235b7b5e',
 
-  'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + 'ffacad996f1f7e38a7f934a7b346ab3671110e48',
+  'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + '2484ae11b5eed2632658081f6d877911517dde2c',
 
   'engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '54c244b9365c602cd31b2be59332d6a1591e2d52',
 

commits.json

diff --git a/commits.json b/commits.json
index c258813..d9cae4a 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "engine/src/flutter":"ffacad996f1f7e38a7f934a7b346ab3671110e48",
+  "engine/src/flutter":"2484ae11b5eed2632658081f6d877911517dde2c",
   "engine/src/flutter/third_party/dart":"54c244b9365c602cd31b2be59332d6a1591e2d52",
   "flutter":"989c14d112b950d7d7ead6f04a9722318430a729"
 }
\ No newline at end of file