Remove legacy architecture stub `UIImplementation` (#56939)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56939

This class was fully stubbed out as part of the Legacy Architecture removal. All public methods had empty bodies or returned null/0, meaning any OSS library still referencing it was already silently broken at runtime.

Also removes the dead `getUIImplementation()` method from `UIManagerModule`, removes `enqueueLayoutUpdateFinished` from `UIViewOperationQueue`, and cleans up stale Javadoc references in `JSTouchDispatcher`, `ReactViewGroup`, and `ReactInterceptingViewGroup`.

`UIBlock`, `NativeViewHierarchyManager`, `UIViewOperationQueue`, and the `addUIBlock`/`prependUIBlock` methods on `UIManagerModule` are retained and will be deprecated in a follow-up diff.

Changelog:
[Android][Removed] - Remove legacy architecture stub `UIImplementation`. This class was already non-functional (all methods were empty stubs).

Reviewed By: javache

Differential Revision: D106086358

fbshipit-source-id: e240b536b11377b7fede4563a09bca1d71f44198