net: remove legacy ethtool ops

As all drivers are converted, we may now remove discrete offload setting
callback handling.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Acked-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>