virtio_net: merge: remove skip_xdp

Now, the logic of merge xdp process is simple, we can remove the
skip_xdp.

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