build(gradle): update build.gradle (#25810)

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: #25810

Test Plan: No tests were run locally

Reviewed By: cpojer

Differential Revision: D16498431

Pulled By: osdnk

fbshipit-source-id: 95e77304549c3f7b7ebdaeb363b72dd21d686595

Author: ferdicus

template/android/build.gradle

@@ -6,7 +6,6 @@ buildscript {
         minSdkVersion = 16
         compileSdkVersion = 28
         targetSdkVersion = 28
-        supportLibVersion = "28.0.0"
     }
     repositories {
         google()