Remove the update-ruby script as it's now unnecessary (#36802)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/36802

As we now support a broader range of ruby version, we don't need `update-ruby.sh` anymore
and we also don't need to call it during the set-rn-version script.

Changelog:
[Internal] [Changed] - Remove the update-ruby script as it's now unnecessary

Reviewed By: cipolleschi

Differential Revision: D44670361

fbshipit-source-id: b23abab28ccbd27408e4d76670101faeb463bc6b