sched/walt: remove references to unused sched_disable_window_stats

sched_disable_window_stats was used while resetting the window
size in legacy code. The current code does not use this anymore.

Change-Id: If77dcfcae917a6632125d205db9082e6b25ccbf6
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>