[MAC80211]: remove key threshold stuff

This patch removes the key threshold stuff from mac80211.
I have patches for later that add it as a per-key setting
to nl/cfg80211.

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