The only user of css_id was memcg, and it has been convered to use
cgroup->id, so kill css_id.
Signed-off-by: NLi Zefan <lizefan@huwei.com>
Reviewed-by: NMichal Hocko <mhocko@suse.cz>
Acked-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NTejun Heo <tj@kernel.org>