Remove unnecessary clean up of targets

- removeTaskInternal can trigger the rotation to change immediately
  (ie. https://paste.googleplex.com/5205159639515136?raw), which will
  inadvertently trigger the screenshotRecentTasks() path, which is
  unnecessary when cleaning up (and can lead to a lingering snapshot
  surface.

  Since these pending new task targets are already added to
  mPendingAnimations when they are created in
  createTaskRemoteAnimation(), the tasks will be cleaned up along with
  everything else in cleanupAnimation()

Bug: 207189813
Test: atest WmTests:RecentsAnimationControllerTest
Test: Ensure the live-tile + rotation path still triggers screenshot
Test: Ensure quickswitch still works
Change-Id: Ia9d2fb01b4c0b205f2fa4615d3be3b2b7070e488