can: remove CAN FD compatibility for CAN 2.0 sockets

In commit e2d265d3 (canfd: add support for CAN FD in CAN_RAW sockets)
CAN FD frames with a payload length up to 8 byte are passed to legacy
sockets where the CAN FD support was not enabled by the application.

After some discussions with developers at a fair this well meant feature
leads to confusion as no clean switch for CAN / CAN FD is provided to the
application programmer. Additionally a compatibility like this for legacy
CAN_RAW sockets requires some compatibility handling for the sending, e.g.
make CAN2.0 frames a CAN FD frame with BRS at transmission time (?!?).

This will become a mess when people start to develop applications with
real CAN FD hardware. This patch reverts the bad compatibility code
together with the documentation describing the removed feature.

Acked-by: Stephane Grosjean <s.grosjean@peak-system.com>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>