net: Kill plain NET_XMIT_BYPASS.

dst_input() was doing something completely absurd, looping
on skb->dst->input() if NET_XMIT_BYPASS was seen, but these
functions never return such an error.

And as a result plain ole' NET_XMIT_BYPASS has no more
references and can be completely killed off.

Signed-off-by: David S. Miller <davem@davemloft.net>