Remove unnecessary unstable_useRuntimeSchedulerAlways Feature Flag (#41158)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/41158

This feature flag was just a killswitch for OSS. As we don't need it anymore, I'm removing it.
I've also discussed with Expo so they remove any usages of this in their codebase.

Changelog:
[Internal] [Changed] - Remove unnecessary unstable_useRuntimeSchedulerAlways Feature Flag

Reviewed By: rubennorte

Differential Revision: D50554334

fbshipit-source-id: b2346654ad543c1350f2f2cae078900abf39d41c