cgroup: remove the superfluous judgment

Remove the superfluous judgment since the function is
never called for a root cgroup, as suggested by Tejun.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Shida Zhang <zhangshida@kylinos.cn>
Reviewed-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>