can: c_can: remove unused code

Commit 9d23a981 ("can: c_can: Remove unused inline function") left
behind C_CAN_MSG_OBJ_TX_LAST constant.

Commit fa39b54c ("can: c_can: Get rid of pointless interrupts") left
behind C_CAN_MSG_RX_LOW_LAST and C_CAN_MSG_OBJ_RX_SPLIT constants.

The removed code also made a comment useless and misleading.

Link: https://lore.kernel.org/r/20210302215435.18286-2-dariobin@libero.it


Signed-off-by: Dario Binacchi <dariobin@libero.it>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>