Avoid checking for window singleton policy when reparenting a window context

This is just a partial revert of Ia97726407b8274c02c32dc68bba9ba85cda84608

the singleton per display check was making each reparenting operation to the default display fail, as the mNotificationShade field from the default display DisplayPolicy didn't wasn't cleaned up after the reparenting happened.

While the proper solution here is cleaning up mNotificationShade from the previous DisplayContent, and setting it in the new one, this is for a follow up cl (to reduce risks of unexpected consequences). This cl only fixes this by partially reverting the recently added check for singleton during reparenting.

Bug: 362719719
Bug: 414579333
Test: atest WindowManagerServiceTest WindowContextistenerControllerTest + Move the shade to an external display, and move it back to the default one.
Flag: com.android.systemui.shade_window_goes_around
Change-Id: I325e5c16d1ad5f2c1aa3ad7d56abda7a9f746efb