Cleans up UserSwitcherActivity.

Now that droidfooders have had access to the refactored
`UserSwitcherActivity` for about two weeks of soak time and no new bugs
have been reported, we can remove the feature flag and clean up all the
dead code.

Bug: 243844097
Test: Ran the activity on tablet, saw that it's working well. Tried
switching users, entering guest mode, navigating away to add users, and
rotating the device.

Change-Id: Ie71bf3f5317e6f994bf33b7898bcf02d27daaa2b