sched: eliminate sched_enable_power_aware knob and parameter

Kill unused scheduler knob and parameter sched_enable_power_aware.  HMP
scheduler always take into account power cost for placing task.

Change-Id: Ib26a21df9b903baac26c026862b0a41b4a8834f3
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>