Delete enableFabricRenderer() from ReactNativeNewArchitectureFeatureFlags (#56860)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56860

The `enableFabricRenderer()` flag is being removed entirely; it was always set to true in the canary release stage. All callers of `ReactNativeNewArchitectureFeatureFlags.enableFabricRenderer()` were inlined in earlier diffs.

This diff:
- Removes the wrapper method `ReactNativeNewArchitectureFeatureFlags.enableFabricRenderer()`.
- Removes the `override fun enableFabricRenderer(): Boolean = true` from `ReactNativeNewArchitectureFeatureFlagsDefaults`.

Both classes are non-`generated`, so they are edited directly. The next diff will remove the underlying `ReactNativeFeatureFlags.enableFabricRenderer()` API by regenerating the codegen output.

Behavior is unchanged.

Changelog:
[Internal]

Reviewed By: javache

Differential Revision: D105231777

fbshipit-source-id: c6fa2e69e1b719ddcb67e61358dfb3a0d4b6307a