btrfs: remove unused member async_submit_bio::fs_info

Introduced by c6100a4b4e3d1 ("Btrfs: replace tree->mapping with
tree->private_data") to be used in run_one_async_done where it got
unused after 736cd52e0c720103 ("Btrfs: remove nr_async_submits and
async_submit_draining").

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -106,7 +106,6 @@ void __cold btrfs_end_io_wq_exit(void)
  */
 struct async_submit_bio {
 	void *private_data;
-	struct btrfs_fs_info *fs_info;
 	struct bio *bio;
 	extent_submit_bio_start_t *submit_bio_start;
 	extent_submit_bio_done_t *submit_bio_done;
@@ -801,7 +800,6 @@ blk_status_t btrfs_wq_submit_bio(struct btrfs_fs_info *fs_info, struct bio *bio,
 		return BLK_STS_RESOURCE;
 
 	async->private_data = private_data;
-	async->fs_info = fs_info;
 	async->bio = bio;
 	async->mirror_num = mirror_num;
 	async->submit_bio_start = submit_bio_start;