cgroup: remove cgroup_mount from comments

Git rid of an outdated comment.

Since cgroup was fully switched to fs_context, cgroup_mount() is gone and
it's confusing to mention in comments of cgroup_kill_sb(). Delete it.
Signed-off-by: zhaoxiaoqiang11 <zhaoxiaoqiang11@jd.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/cgroup.c

@@ -2169,7 +2169,6 @@ static void cgroup_kill_sb(struct super_block *sb)
 	/*
 	 * If @root doesn't have any children, start killing it.
 	 * This prevents new mounts by disabling percpu_ref_tryget_live().
-	 * cgroup_mount() may wait for @root's release.
 	 *
 	 * And don't kill the default root.
 	 */