can: gw: remove obsolete checks

In commit be286baf ("can: gw: add a variable
limit for CAN frame routings") the detection of the frame routing has been
changed. The former solution required dev->header_ops to be unused (== NULL).

I missed to remove the obsolete checks in the original commit - so here it is.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>