cgroup: remove unused macro for_each_e_css()

for_each_e_css() is unused now. Remove it.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>