Optimise apply_slack() for size and speed

Change-Id: I7e330491dfcfd22cf05bee234298a93c9d346e5b
sched: Kill WAKEUP_PREEMPT

Remove the WAKEUP_PREEMPT feature, disabling it doesn't make any sense
and its outlived its use by a long long while.

Change-Id: I36e5e74af9c6f5d32b92e5334c4f735a39fe962c