Clean up some dead code.

The close transition only be create in startPostCommitAnimation, so it
won't happen before startSystemAnimation. Also once the timeout happen,
shell will ask core to cancel the animation, so there won't have
"transition before animation" case.

Bug: 347168362
Flag: com.android.window.flags.migrate_predictive_back_transition
Test: atest BackAnimationControllerTest
Change-Id: Ice26762bd0c7eb8a7cdf5d6b010abdd9c1030d95