virtio-net: remove unused skb_vnet_hdr->num_sg field

[Split from "correct capacity math on ring full" -- Rusty]

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Michael S. Tsirkin <mst@redhat.com>