Clean up wifi configuration some more

* Remove /data/system/wpa_supplicant/wlan0 symlink hack
* Do not create the /data/system/wpa_supplicant directory as it is no
  longer needed and triggers some backwards compatibility code.
* Start wpa_supplicant with the correct global socket name
* Remove local copy of wpa_supplicant.conf - it is identical to the
  version installed by default.