mm/memcg: update obsolete comment above parent_mem_cgroup()

Since commit bef8620c ("mm: memcg: deprecate the non-hierarchical
mode"), use_hierarchy is already deprecated.  And it's further removed via
commit 9d9d341d

 ("cgroup: remove obsoleted broken_hierarchy and
warned_broken_hierarchy").  Update corresponding comment.

Link: https://lkml.kernel.org/r/20230801124359.2266860-1-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>