Removes FlaggedApi for ACCESSIBILITY_MOTION_EVENT_OBSERVING

This flag annotation is referencing the wrong string;
the real flag android.view.accessibility.motion_event_observing
is already rolled out.

Bug: 378545161
Flag: android.view.accessibility.motion_event_observing
Test: n/a
Change-Id: Ic563c048674712105697b264f8e5e8375bd3fc8b