cgroup: cls: remove unnecessary task_cls_classid

We can get classid through cgroup_subsys_state,
this is directviewing and effective.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>