drivers: can: remove CAN_HAS_RX_TIMESTAMP Kconfig helper

Remove the CAN_HAS_RX_TIMESTAMP Kconfig helper symbol in order to allow
enabling CAN RX timestamps in the API regardless of driver support.

This simplifies application prj.conf settings across board supporting/not
supporting RX timestamps considerably.

CAN drivers not supporting RX timestamps already initialize the timestamp
to 0 for received frames.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>