mac80211: remove offchannel_tx API

For iwlwifi, I decided not to use this API since
it just increased the complexity for little gain.
Since nobody else intends to use it, let's kill
it again. If anybody later needs to have it, we
can always revive it then.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>