fix system_flag(scheduling_statistics,disable)

Clear the "enabled" flag for scheduling statistics when disable is
specified.
Merge branch 'vinoski/sv/fix-sched-stats-disable/OTP-11317' into maint

* vinoski/sv/fix-sched-stats-disable/OTP-11317:
  fix system_flag(scheduling_statistics,disable)
Merge branch 'maint'