virtio: Remove the unused poll method

This was used by virtio_mmio to poll via another driver. This support
has been removed so we can remove it from here too.

Reviewed by:	br
Sponsored by:	Arm Ltd
Differential Revision:	https://reviews.freebsd.org/D51993

sys/dev/virtio/virtio_bus_if.m

@@ -109,7 +109,3 @@ METHOD void write_device_config {
 	int		len;
 };
 
-METHOD void poll {
-	device_t	dev;
-};
-

sys/dev/virtio/virtqueue.c

@@ -605,10 +605,8 @@ virtqueue_poll(struct virtqueue *vq, uint32_t *len)
 {
 	void *cookie;
 
-	VIRTIO_BUS_POLL(vq->vq_dev);
 	while ((cookie = virtqueue_dequeue(vq, len)) == NULL) {
 		cpu_spinwait();
-		VIRTIO_BUS_POLL(vq->vq_dev);
 	}
 
 	return (cookie);