Revert "sched: Use only partial wait time as task demand"

This reverts commit 0e2092e4 ("sched: Use only partial wait time as
task demand") as it causes performance regression.

Change-Id: I3917858be98530807c479fc31eb76c0f22b4ea89
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>