Fix flicker

This line was added for window swapping, but it's not actually needed
and causes regressions.

Bug: 27635189
Change-Id: I6c9cdac8d1bf30029b9f36a106e1f2afebfd9072

services/core/java/com/android/server/am/ActivityStack.java

@@ -1871,7 +1871,6 @@ final class ActivityStack {
             // appropriate for it.
             mStackSupervisor.mStoppingActivities.remove(r);
             mStackSupervisor.mGoingToSleepActivities.remove(r);
-            mStackSupervisor.mWaitingVisibleActivities.remove(r);
         } catch (Exception e) {
             // Just skip on any failure; we'll make it
             // visible when it next restarts.