Fix/simplify app removal code.

1. Remove a remnant of the original implementation that assumed apps
   were holding credits to the detriment of others and needed to be
   added back to the main pool.
2. Operate at the user level when a user is removed to remove
   unnecessary looping.

Bug: 242899307
Test: atest com.android.cts.net.HostsideRestrictBackgroundNetworkTests --use-modules-in
Test: atest frameworks/base/services/tests/mockingservicestests/src/com/android/server/tare
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/tare
Change-Id: I6b76a0d652af4a1a248ff5cb29526bef0e9d221d