network: Remove explicit autoconnection code

NM is now a lot smarter about dealing with automatic connections, so just
create an empty connection and pass it to it. The only places where NM
requires connection settings is where we require explicit setup: Bluetooth
DUN, WPA-Enterprise and WWAN/VPN. These cases are already handled by
gnome-control-center, where complex configuration is handled, so remove
the automatic connection management for now and just let NM handle it.

https://bugzilla.gnome.org/show_bug.cgi?id=698918