can: c_can: remove unused enum BOSCH_C_CAN_PLATFORM

This patch removes the unused enum BOSCH_C_CAN_PLATFORM.

Link: https://lore.kernel.org/r/20210406110617.1865592-2-mkl@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>