sched: remove call of sched_avg_update from sched_rt_avg_update

rt_avg is only used to scale the available CPU's capacity for CFS
tasks.  As the update of this scaling is done during periodic load
balance, we only have to ensure that sched_avg_update has been called
before any periodic load balancing. This requirement is already
fulfilled by __update_cpu_load so the call in sched_rt_avg_update,
which is part of the hotpath, is useless.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Steve Muckle <smuckle@linaro.org>