btrfs: remove unused member list from async_submit_bio

The list used to track checksums in the early version (2.6.29), but I
was able not pinpoint the commit that stopped using it. Everything
apparently works without it for a long time.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -121,7 +121,6 @@ struct async_submit_bio {
 	void *private_data;
 	struct btrfs_fs_info *fs_info;
 	struct bio *bio;
-	struct list_head list;
 	extent_submit_bio_hook_t *submit_bio_start;
 	extent_submit_bio_hook_t *submit_bio_done;
 	int mirror_num;