Remove enableMainQueueCoordinatorOnIOS, make coordinator always-on (#56935)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56935

The `enableMainQueueCoordinatorOnIOS` flag has finished rollout. Drop it from the feature-flag config, regenerate the per-language accessors, and inline the previously-gated paths:

- `RCTUtils.mm`: `RCTUnsafeExecuteOnMainQueueSync` and the `RCTUnsafeExecuteOnMainQueueOnceSync` helper always use `unsafeExecuteOnMainThreadSync` on iOS (the TV fallback to `dispatch_sync(main, ...)` stays).
- `RuntimeExecutorSyncUIThreadUtils.mm`: `executeSynchronouslyOnSameThread_CAN_DEADLOCK` switches between two paths based on the calling thread instead of the feature flag. Main-thread callers (production) take the coordinator path that pumps UI tasks while waiting for JS; off-main callers (e.g. RuntimeScheduler unit tests) take a simpler path that just waits for the runtime and runs `runtimeWork` synchronously on the calling thread. Trimmed the `runtimeCaptureBlockDone` plumbing that an old TODO already flagged as unnecessary.

Also drops the iOS override in the Wilde plugin, the now-dead `rn_fling.enable_main_queue_coordinator_on_ios` MobileConfig param, and the `flagOff` variant of the characterization test (the on-path test still pins the contract).

Changelog:
[iOS][Changed] - `RCTUnsafeExecuteOnMainQueueSync` and bridgeless sync runtime-thread calls now always use the coordinator implementation that pumps UI tasks while waiting for JS, eliminating a class of deadlocks. The previous opt-in flag has been removed.

Reviewed By: sammy-SC

Differential Revision: D105984547

fbshipit-source-id: 1eee1b6e5875b108e145fdd295dd3951ba1d839b