sched: Remove sched_schedstats sysctl out from under SCHED_DEBUG

CONFIG_SCHEDSTATS does not depend on SCHED_DEBUG, it is inconsistent
to have the sysctl depend on it.

Suggested-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20210412102001.161151631@infradead.org