Remove onBackInvoked workaround for recents animation.

Legacy recents animation was broken for a while after enable shell
transition, it's unnecessary to maintain it.
For shell transition, after introduce RecentsTransitionHandler, it
will set focus window to recents activity in setInputConsumerEnabled
(ref I10e397db956cad90a4510c326163600f0ed63514), so onBackCallback can
handled by focus window directly.

Bug: 333428882
Test: tried back gesture work in recents when either enable/disable
enable_predictive_back_gesture.

Change-Id: I2e2d6dc7adf0dd9b821b65ff328e434c2600bcf5