Revert "[vm/ffi] Add class modifiers"

This reverts commit 1755f8909264330a207a0e0e44d31f5fec9c489a.

Reason for revert: This is a breaking change and is blocking the Dart -> Flutter roll. See https://ci.chromium.org/ui/p/flutter/builders/try/Mac%20Unopt/26896/overview.

Original change's description:
> [vm/ffi] Add class modifiers
>
> Adds class modifiers to `dart:ffi`.
>
> Migrates all user-defined subclasses of `Struct`, `Union`, `Opaque`,
> and `AbiSpecificInteger` to be `final class`es.
>
> Does not remove the manual error checking, so some errors will show up
> twice now in language version 3.0. In language version <3.0, only the
> FFI-specific error will show up.
>
> In a follow-up CL, we will try to make the language-errors to show up
> also <3.0 so that we can remove the FFI-specific errors.
>
> Examples of duplicated errors:
> pkg/analyzer/test/src/diagnostics/subtype_of_ffi_class_test.dart
>
> TEST=pkg/analyzer/test/ (for the analyzer)
> TEST=pkg/front_end/testcases/ (for the CFE)
> TEST=test/ffi/ (for the VM)
>
> CoreLibraryReviewExempt: No need for dart2js to review.
> Bug: https://github.com/dart-lang/sdk/issues/51683
> Change-Id: I2964ceccb7db59fbdaf6be5319f5e4ec2dabe0f3
> Cq-Include-Trybots: luci.dart.try:pkg-linux-debug-try,pkg-win-release-try,pkg-mac-release-try,vm-precomp-ffi-qemu-linux-release-riscv64-try,vm-precomp-ffi-qemu-linux-release-arm-try,vm-ffi-android-debug-arm64c-try,vm-ffi-android-debug-arm-try,vm-reload-rollback-linux-debug-x64-try,analyzer-analysis-server-linux-try,analyzer-linux-release-try
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/289223
> Reviewed-by: Johnni Winther <johnniwinther@google.com>
> Reviewed-by: Devon Carew <devoncarew@google.com>
> Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
> Reviewed-by: Jackson Gardner <jacksongardner@google.com>
> Reviewed-by: Lasse Nielsen <lrn@google.com>
> Commit-Queue: Daco Harkes <dacoharkes@google.com>

Bug: https://github.com/dart-lang/sdk/issues/51683
Change-Id: Ie5b8a08aea6d64b1991ace4814322b21ffb670c7
Cq-Include-Trybots: luci.dart.try:pkg-linux-debug-try,pkg-win-release-try,pkg-mac-release-try,vm-precomp-ffi-qemu-linux-release-riscv64-try,vm-precomp-ffi-qemu-linux-release-arm-try,vm-ffi-android-debug-arm64c-try,vm-ffi-android-debug-arm-try,vm-reload-rollback-linux-debug-x64-try,analyzer-analysis-server-linux-try,analyzer-linux-release-try
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/289640
Commit-Queue: Slava Egorov <vegorov@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Alexander Aprelev <aam@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>