sched: eliminate sched_account_wait_time knob

Kill unused scheduler knob sched_account_wait_time.  With this change
scheduler always accounts task's wait time into demand.

Change-Id: Ifa4bcb5685798f48fd020f3d0c9853220b3f5fdc
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>