nfp: flower: remove unnecessary code in flow lookup

Recent changes to NFP mean that stats updates from fw to driver no longer
require a flow lookup and (because egdev offload has been removed) the
ingress netdev for a lookup is now always known.

Remove obsolete code in a flow lookup that matches on host context and
that allows for a netdev to be NULL.

Signed-off-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>