virtio_balloon: drop legacy_only driver flag

we have blacklisted balloon in core, no need
for a driver flag.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>

drivers/virtio/virtio_balloon.c

@@ -518,7 +518,6 @@ static unsigned int features[] = {
 };
 
 static struct virtio_driver virtio_balloon_driver = {
-	.legacy_only = true,
 	.feature_table = features,
 	.feature_table_size = ARRAY_SIZE(features),
 	.driver.name =	KBUILD_MODNAME,