Revert "network: Use NMDevice.get_available_connections(); for NMWirelessDialog"

This reverts commit a36bfced473d48ffe70150ad5537bb82b1ead8e8.

get_available_connections() returns the connections that are
available right now, ie, with the currently visible APs, but
that might change while the dialog is open, so we can't use it.

https://bugzilla.gnome.org/show_bug.cgi?id=709128