libertas: remove some unused commands

Neither CMD_802_11_PAIRWISE_TSC nor CMD_802_11_GROUP_TSC is used or
documented. It might have something to do with TKIP sequence counters,
but that's just an educated guess. Remove all occurences of them.

CMD_CODE_DNLD is also neither used nor documented.

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>