Clean up postWindowRemoveStartingWindowCleanUp

The starting window will be remove automatically when activity died,
so there doesn't need check whether the activity need to remove
starting window when a window gone.
When the client process killed during launch, the core can keep
starting window on screen since there will restart the process with
original activity, so doesn't need to remove starting window.

Bug: 280516527
Bug: 290203002
Test: verify no regression for 156298440
Test: simulate to kill top app process while launch app, and verify no
flickering occur.

Change-Id: If85386e963d7cca0dad9a143d6de0e7cd87b05f0