Revert "Add android.system.Os wrappers for getgroups/setgroups"

These (@hide) methods are unused, and the team who initially requested
them no longer need them. Also, the tests for setgroups recently
started failing (b/77599196).

This reverts commit 5bc6ac72.

Bug: 77599196
Test: Treehugger
Change-Id: I7da6c86d090c1c1f6f8ecbc21eb447aff38649b8