mm/memcg: update obsolete comment above parent_mem_cgroup()

Since commit bef8620 ("mm: memcg: deprecate the non-hierarchical
mode"), use_hierarchy is already deprecated.  And it's further removed via
commit 9d9d341 ("cgroup: remove obsoleted broken_hierarchy and
warned_broken_hierarchy").  Update corresponding comment.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Miaohe Lin <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Muchun Song <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: MiaoheLin

include/linux/memcontrol.h

@@ -860,8 +860,7 @@ static inline struct mem_cgroup *lruvec_memcg(struct lruvec *lruvec)
  * parent_mem_cgroup - find the accounting parent of a memcg
  * @memcg: memcg whose parent to find
  *
- * Returns the parent memcg, or NULL if this is the root or the memory
- * controller is in legacy no-hierarchy mode.
+ * Returns the parent memcg, or NULL if this is the root.
  */
 static inline struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *memcg)
 {