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>

kernel/cgroup/cgroup.c

@@ -5685,7 +5685,7 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
 	css_clear_dir(&cgrp->self);
 	kernfs_remove(cgrp->kn);
 
-	if (parent && cgroup_is_threaded(cgrp))
+	if (cgroup_is_threaded(cgrp))
 		parent->nr_threaded_children--;
 
 	spin_lock_irq(&css_set_lock);