virtio_net: remove ndo_xdp_flush call virtnet_xdp_flush

Remove the ndo_xdp_flush call implementation virtnet_xdp_flush
as no callers of ndo_xdp_flush are left.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>