sched: remove redundant call to unregister_sched_domain_sysctl()

Impact: cleanup

The sysctl has been unregistered by partition_sched_domains().

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>