writeback: remove unused delaration of bdi_async_bio_wq

It seems it was introduced by commit d3f77dfdc718 ("blkcg: implement
REQ_CGROUP_PUNT") unintentionally, but the definition does not exist,
remove it.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Xiu Jianfeng <[e-mail]>
Acked-by: Tejun Heo <[e-mail]>
Cc: Stefan Roesch <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/backing-dev.h

···
 extern struct list_head bdi_list;
 
 extern struct workqueue_struct *bdi_wq;
-extern struct workqueue_struct *bdi_async_bio_wq;
 
 static inline bool wb_has_dirty_io(struct bdi_writeback *wb)
 {