Remove DepartingAnimationTarget from BackMotionEvent

This would create another SurfaceControl object through IPC, which is
unnecessary because the animation should run through
BackAnimationRunner.

Flag: EXEMPT bugfix
Bug: 395035430
Test: execute back-to-home transition several times, verify no
predict_back animation leash left in offscreen layer.

Change-Id: I6a54327e28576b5755b5b2536c6c4eb37712c56a