Cleanup prealpha logic from the JS Publishing Scripts (#48691)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/48691

We don't intent to use the prealpha logic in the near future so it makes sense to remove it for
to simplify our already complicated release process. We can always revive it if we wish.

Changelog:
[Internal] [Changed] -

Reviewed By: cipolleschi

Differential Revision: D68206014

fbshipit-source-id: f05eeae3997d52df1127852e03437a387a01f5ad