sched: Remove unused code in sched_avg.c

The iowait tasks running average is computed but not presented
to core_ctl. Remove this unused code to save a multiplication
operation during enqueue/dequeue.

Change-Id: Ida51b049977d4a33a0bf42fe297a648362d5a317
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>