virtio_net: small: remove the delta

In the case of XDP-PASS, skb_reserve uses the "delta" to compatible
non-XDP, now that is not shared between xdp and non-xdp, so we can
remove this logic.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>