can: remove obsolete pernet_operations definitions

The namespace support for the CAN subsystem does not need any additional
memory. So when ".size = 0" there's no extra memory allocated by the system.
And therefore ".id" is obsolete too.

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