btrfs: simplify btree_submit_bio_start and btrfs_submit_bio_start parameters

After previous patches the unused parameters can be removed from
btree_submit_bio_start and btrfs_submit_bio_start as they don't need to
conform to the extent_submit_bio_start_t typedef.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>