btrfs: Cleanup the unused struct async_sched.

The struct async_sched is not used by any codes and can be removed.
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Reviewed-by: Josef Bacik <jbacik@fusionio.com>
Tested-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fb.com>

fs/btrfs/volumes.c

@@ -5323,13 +5323,6 @@ static void btrfs_end_bio(struct bio *bio, int err)
 	}
 }
 
-struct async_sched {
-	struct bio *bio;
-	int rw;
-	struct btrfs_fs_info *info;
-	struct btrfs_work work;
-};
-
 /*
  * see run_scheduled_bios for a description of why bios are collected for
  * async submit.