cgroup: Remove unused cgrp variable

The 'cgrp' is set but not used in commit <76f969e8>
("cgroup: cgroup v2 freezer").
Remove it to avoid [-Wunused-but-set-variable] warning.

Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Acked-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Tejun Heo <tj@kernel.org>