mac80211: remove mixed-cell and userspace MLME code

Neither can currently be set from userspace, so there's no
regression potential, and neither will be supported from
userspace since the new userspace APIs allow the SME, which
is in userspace, to control all we need.

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