mm: memcg: remove obsolete memcg_has_children()

Commit 2ef1bf118c40 ("mm: memcg: deprecate the non-hierarchical mode")
removed the only use of memcg_has_children() in
mem_cgroup_hierarchy_write() as part of the feature deprecation.

Hence, since then, make CC=clang W=1 warns:

  mm/memcontrol.c:3421:20: warning: unused function 'memcg_has_children' [-Wunused-function]

Simply remove this obsolete unused function.

Link: https://lkml.kernel.org/r/20201116055043.20886-1-lukas.bulwahn@gmail.comSigned-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Roman Gushchin <guro@fb.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -3453,19 +3453,6 @@ unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
 	return nr_reclaimed;
 }
 
-/*
- * Test whether @memcg has children, dead or alive.
- */
-static inline bool memcg_has_children(struct mem_cgroup *memcg)
-{
-	bool ret;
-
-	rcu_read_lock();
-	ret = css_next_child(NULL, &memcg->css);
-	rcu_read_unlock();
-	return ret;
-}
-
 /*
  * Reclaims as many pages from the given memcg as possible.
  *