Revert "sched: use cum_window_demand for rt task placement"

This reverts commit 50543957.

The idea of RT task packing isn't great due to potential performance
disadvantage.  Revert the commit so we can find the least load CPU
in the lowest possible capacity CPU.

Change-Id: I6495e36549a7ed610f0b69697ddc44c79ca600c8
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>