sched: window-stats: Code cleanup

add_task_demand() and 'long_sleep' calculation in it are not strictly
required. rq_freq_margin() check for need to change frequency, which
removes need for long_sleep calculation. Once that is removed, need
for add_task_demand() vanishes.

Change-Id: I936540c06072eb8238fc18754aba88789ee3c9f5
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>