btrfs: remove unused member async_submit_bio::fs_info

Introduced by c6100a4b ("Btrfs: replace tree->mapping with
tree->private_data") to be used in run_one_async_done where it got
unused after 736cd52e ("Btrfs: remove nr_async_submits and
async_submit_draining").

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