cgroup: Remove unnecessary list_empty()

The root hasn't been removed from the root_list, so the list can't be NULL.
However, if it had been removed, attempting to destroy it once more is not
possible. Let's replace this with WARN_ON_ONCE() for clarity.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>