cfg80211/nl80211: remove usage of CONFIG_NL80211

The scan capability added to cfg80211/nl80211 introduced a
dependency on nl80211 by cfg80211. We can thus no longer have
just cfg80211 without nl80211. Specifically, cfg80211_scan_done()
calls nl80211_send_scan_aborted() or nl80211_send_scan_done().

Now we remove the option for user to select nl80211. It will always
be compiled if user selects cfg80211.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>