net: Remove ndo_dfwd_start_xmit

Looks like commit f663dd9a ("net: core: explicitly select a txq before doing l2 forwarding")
has removed the need for this dedicated xmit function [it even explicitly
states so in its commit log message] but it hasn't removed the definition
of the ndo.

Signed-off-by: Yuval Mintz <Yuval.Mintz@cavium.com>
CC: Jason Wang <jasowang@redhat.com>
CC: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>