network: Remove setActiveConnection/clearActiveConnection

network: Remove setActiveConnection/clearActiveConnection

This can be more easily achieved by listening for changes to the
device's active-connection property. VPN will still need support to
track active connections, as it does not have an associated
device. But as VPN can track multiple active connections, the names
"set" and "clear" don't quite fit. Rename them to the more-standard
"add" and "remove".

https://bugzilla.gnome.org/show_bug.cgi?id=701954