virtio: remove obsolete virtqueue_get_queue_index()

You can access it directly now, since 3.8: v3.7-rc1-13-g06ca287
'virtio: move queue_index and num_free fields into core struct
virtqueue.'

Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>