writeback: cleanup bdi_sched_wait()

bdi_sched_wait() is no longer used since commit 839a8e8 ("writeback:
replace custom worker pool implementation with unbound workqueue"), so
remove it.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Xiu Jianfeng <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Reviewed-by: Johannes Thumshirn <[email protected]>
Acked-by: Jens Axboe <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Xiu Jianfeng

include/linux/backing-dev.h

@@ -140,12 +140,6 @@ static inline bool mapping_can_writeback(struct address_space *mapping)
 	return inode_to_bdi(mapping->host)->capabilities & BDI_CAP_WRITEBACK;
 }
 
-static inline int bdi_sched_wait(void *word)
-{
-	schedule();
-	return 0;
-}
-
 #ifdef CONFIG_CGROUP_WRITEBACK
 
 struct bdi_writeback *wb_get_lookup(struct backing_dev_info *bdi,