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Git rid of an outdated comment.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>