writeback: remove dead code in wb_blkcg/memcg_offline

Signed-off-by: Jiufei Xue <jiufei.xue@linux.alibaba.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

mm/backing-dev.c

@@ -745,7 +745,6 @@ static void cgwb_bdi_unregister(struct backing_dev_info *bdi)
  */
 void wb_memcg_offline(struct mem_cgroup *memcg)
 {
-	LIST_HEAD(to_destroy);
 	struct list_head *memcg_cgwb_list = mem_cgroup_cgwb_list(memcg);
 	struct bdi_writeback *wb, *next;
 
@@ -764,7 +763,6 @@ void wb_memcg_offline(struct mem_cgroup *memcg)
  */
 void wb_blkcg_offline(struct blkcg *blkcg)
 {
-	LIST_HEAD(to_destroy);
 	struct bdi_writeback *wb, *next;
 
 	spin_lock_irq(&cgwb_lock);