mac80211: remove STA infos last_ack stuff

These things aren't used and the only possible use is within
rate control algorithms, however those can, if they need it,
keep track of it in their private data. last_ack_ms isn't
even updated so completely useless.

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