Fling: Cleanup `enableVirtualViewExperimental` Feature Flag (#55065)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/55065

Cleans up the `enableVirtualViewExperimental` feature flag, so that only the experimental one is ever used.

This does not delete the original implementation or rename the experimental one. I'll do those separately as follow-ups.

Changelog:
[Internal]

Reviewed By: lunaleaps

Differential Revision: D90193651

fbshipit-source-id: e64a40b8ed49e4420370da300f03a8576a49321d