[Multi-user] Clean up user state stored in the system user directory

when user is removed.

For non system users, backup state is stored in both the user's own dir and the system dir.
When the user is removed, the user's own dir gets removed by the OS. This code change ensures
that the part of the user backup state which is in the system dir also gets removed.

Bug: 127650374

Test: atest -v CtsBackupHostTestCases:android.cts.backup.MultiUserBackupStateTest

Change-Id: I4ea252e8e6da608e36ec3ac335666923d88a8748