s390/qeth: remove OSN support

Commit fb64de1b

 ("s390/qeth: phase out OSN support") spelled out
why the OSN support in qeth is in a bad shape, and put any remaining
interested parties on notice to speak up before it gets ripped out.

It's 2021 now, so make true on that promise and remove all the
OSN-specific parts from qeth. This also means that we no longer need to
export various parts of the cmd & data path internals to the L2 driver.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>