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>

kernel/cgroup/cgroup.c

@@ -5864,11 +5864,8 @@ void cgroup_post_fork(struct task_struct *child)
 		 * the task into the frozen state.
 		 */
 		if (unlikely(cgroup_task_freeze(child))) {
-			struct cgroup *cgrp;
-
 			spin_lock(&child->sighand->siglock);
 			WARN_ON_ONCE(child->frozen);
-			cgrp = cset->dfl_cgrp;
 			child->jobctl |= JOBCTL_TRAP_FREEZE;
 			spin_unlock(&child->sighand->siglock);