cpuset: remove fast exit path from remove_tasks_in_empty_cpuset()

The function isn't that hot, the overhead of missing the fast exit is
low, the test itself depends heavily on cgroup internals, and it's
gonna be a hindrance when trying to decouple cpuset locking from
cgroup core.  Remove the fast exit path.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

kernel/cpuset.c

@@ -1967,14 +1967,6 @@ static void remove_tasks_in_empty_cpuset(struct cpuset *cs)
 {
 	struct cpuset *parent;
 
-	/*
-	 * The cgroup's css_sets list is in use if there are tasks
-	 * in the cpuset; the list is empty if there are none;
-	 * the cs->css.refcnt seems always 0.
-	 */
-	if (list_empty(&cs->css.cgroup->css_sets))
-		return;
-
 	/*
 	 * Find its next-highest non-empty parent, (top cpuset
 	 * has online cpus, so can't be empty).