sched/rt: code cleanup, remove a redundant function call

The second time call to sched_rt_period is redundant, because the value of the
rt_runtime was already read and it was protected by the rurt_runtime_lock.

Signed-off-by: Shan Hai <haishan.bai@gmail.com>
Signed-off-by: nobodyAtall <tsogkas@ceid.upatras.gr>
Signed-off-by: garwedgess <garethwilliams21@gmail.com>