Revert "Prevent removing the target user during a user switch."

This reverts commit f9e7fdc3.

Reason for revert: We need to prevent removing both target and source users during a user switch. b/264999003#comment2
ag/20948004 will be submitted instead of this one.

Change-Id: Ie3bc4c72033fe3d2edaa5800a3c9d445002ef758