virtio_net: small: remove skip_xdp

Because the skb build code is not shared between xdp and non-xdp, and
the xdp code in receive_small() is simpler, so "skip_xdp" is not needed.
We can remove it.

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