sched: remove sched_new_task_windows tunable

The sched_new_task_windows tunable is set to 5 in the scheduler
and it is not changed from user space. Remove this unused tunable.

Change-Id: I771e12b44876efe75ce87a90e4e9d69c22168b64
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>