[MAC80211]: remove set_key_idx callback

No existing drivers use this callback, hence there's no telling
how it might be used. In fact, it is unlikely to be of much use
as-is because the default key index isn't something that the
driver can do much with without knowing which interface it was
for etc. And if it needs the key index for the transmitted frame,
it can get it by keeping a reference to the key_conf structure
and looking it up by hw_key_idx.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Michael Wu <flamingice@sourmilk.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>