mac80211: remove dead code, clean up

With mac80211 now always controlled by an external SME,
a lot of code is dead -- SSID, BSSID, channel selection
is always done externally, etc. Additionally, rename
IEEE80211_STA_TKIP_WEP_USED to IEEE80211_STA_DISABLE_11N
and clean up the code a bit.

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