] Clean up feature flag to enable paint time reporting in Event Timing API (#50698)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/50698

Changelog: [internal]

This cleans up the flag to enable paint time reporting for Event Timing API entries, as this reporting fixes a bug and we've verified is stable/performant enough.

Reviewed By: rshest

Differential Revision: D72960337

fbshipit-source-id: 976810382208b9403c01c80324b540570bbc0e42