mac80211: remove tx status ampdu_ack_map

There's a single use of this struct member, but
as it is write-only it clearly not necessary.
Thus we can free up some space here, even if we
don't need it right now it seems pointless to
carry around the variable.

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