timer: remove clockevents_unregister_notifier

I find a function(clockevents_unregister_notifier) which is not called by
anything in tree.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/time/clockevents.c

@@ -113,16 +113,6 @@ int clockevents_register_notifier(struct notifier_block *nb)
 	return ret;
 }
 
-/**
- * clockevents_unregister_notifier - unregister a clock events change listener
- */
-void clockevents_unregister_notifier(struct notifier_block *nb)
-{
-	spin_lock(&clockevents_lock);
-	raw_notifier_chain_unregister(&clockevents_chain, nb);
-	spin_unlock(&clockevents_lock);
-}
-
 /*
  * Notify about a clock event change. Called with clockevents_lock
  * held.