Revert "Add getgroups()/setgroups() to libcore.Os"

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 44177e69.

Bug: 77599196
Test: Treehugger
Change-Id: Ia22ae6ab91503b221dd9fc8232c6fe651b25689e