Summary:
Remove obsolete supportLibVersion - no longer required with AndroidX
Removing the `supportLibVersion` variable that was used in `app/build.gradle` to suffix version numbers on android support libraries.
As react-native moved to androidX with the release of `0.60` this is no longer necessary.
## Changelog
[Android] [Removed] - Remove supportLibVersion variable in build.gradle
Pull Request resolved: https://github.com/facebook/react-native/pull/25810
Test Plan: No tests were run locally
Reviewed By: cpojer
Differential Revision: D16498431
Pulled By: osdnk
fbshipit-source-id: 95e77304549c3f7b7ebdaeb363b72dd21d686595
fee7f0617e