UserRemovalWaiter - Stop querying alive users in addition to broadcast

In UserRemovalWaiter, in addition to waiting for ACTION_USER_REMOVED
broadcast, alive users were queried every one second, in case the
broadcast was delayed. But it turns out querying users is adding
unnecessary complexity and making the test less stable.
This CL reverts that experimental change.

Bug: 264534887
Test: atest com.android.server.pm.UserManagerTest
Change-Id: I20b9b1d3e808706336ba260192c8ea02eba49658