sched/core: Skip superfluous acquire barrier in ttwu

ttwu_remote() unconditionally locks the task's runqueue lock, which implies
a full barrier across the lock and unlock, so the acquire barrier after the
control dependency is only needed when the task isn't on the runqueue.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
Change-Id: Ibb988fe341ba3109d381682a8725fbd2e7a648e3