Remove `fixFindShadowNodeByTagRaceCondition` feature flag (#56850)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56850

The `fixFindShadowNodeByTagRaceCondition` feature flag has been fully rolled out on both iOS and Android. This change removes the flag and makes the fixed behavior the permanent default.

The fix (https://github.com/facebook/react-native/pull/55751) replaced a use-after-free race condition in `findShadowNodeByTag_DEPRECATED` where a raw pointer was captured via `tryCommit()` and could dangle after a concurrent surface stop. The fixed code path uses `getCurrentRevision()` which returns a `shared_ptr` that keeps the root alive for the entire traversal.

Changelog: [Internal]

Reviewed By: sammy-SC

Differential Revision: D105322626

fbshipit-source-id: 018a6a3646bbca9e8f7854848a8a3ea5f2dbf1d0