[MAC80211]: remove IEEE80211_CONF_SSID_HIDDEN

The IEEE80211_CONF_SSID_HIDDEN setting is not useful for any driver
we have and should be a per-interface setting anyway. Remove it.

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>