Commit [1] removed the need to use btrfs_async_submit_limit(), so
delete it.

[1]
 commit 736cd52e
  Btrfs: remove nr_async_submits and async_submit_draining
Signed-off-by: NAnand Jain <anand.jain@oracle.com>
Reviewed-by: NNikolay Borisov <nborisov@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>