can: af_can: can_pernet_exit(): no need to iterate over and cleanup registered CAN devices

The networking core takes care and unregisters every network device in
a namespace before calling the can_pernet_exit() hook. This patch
removes the unneeded cleanup.

Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Suggested-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>