writeback: remove unused delaration of bdi_async_bio_wq

It seems it was introduced by commit d3f77dfd ("blkcg: implement
REQ_CGROUP_PUNT") unintentionally, but the definition does not exist,
remove it.

Link: https://lkml.kernel.org/r/20230812110128.482650-1-xiujianfeng@huaweicloud.com


Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Stefan Roesch <shr@devkernel.io>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>