Remove RSNRU feature flag after holdout cleanup (#49146)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49146

Runtime Reference ShadowNode Update is enabled by default and no longer referenced in the RN holdout. This diff:
- removes the feature flag
- removes all references to the feature flag, enabling it
- updates the unit test for RSNRU

Changelog: [Internal]

Reviewed By: javache

Differential Revision: D69061234

fbshipit-source-id: 0dab0b5cae99e83297f34645dee58ae9b3c0dc5f