virtio: get rid of redundant VIRTIO_ID_9P definition

VIRTIO_ID_9P is already defined in include/linux/virtio_9p.h
so use that definition instead.
Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Eric Van Hensbergen <ericvh@gmail.com>

net/9p/trans_virtio.c

@@ -334,8 +334,6 @@ static void p9_virtio_remove(struct virtio_device *vdev)
 	}
 }
 
-#define VIRTIO_ID_9P 9
-
 static struct virtio_device_id id_table[] = {
 	{ VIRTIO_ID_9P, VIRTIO_DEV_ANY_ID },
 	{ 0 },