mm/memcontrol.c: remove redundant BUG_ON() in mem_cgroup_usage_unregister_event()

In the following code:

	if (type == _MEM)
		thresholds = &memcg->thresholds;
	else if (type == _MEMSWAP)
		thresholds = &memcg->memsw_thresholds;
	else
		BUG();

	BUG_ON(!thresholds);

The BUG_ON() seems redundant.
Signed-off-by: NAnton Vorontsov <anton.vorontsov@linaro.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -4467,12 +4467,6 @@ static void mem_cgroup_usage_unregister_event(struct cgroup *cgrp,
 	else
 		BUG();
 
-	/*
-	 * Something went wrong if we trying to unregister a threshold
-	 * if we don't have thresholds
-	 */
-	BUG_ON(!thresholds);
-
 	if (!thresholds->primary)
 		goto unlock;