Much as with nm-remote-settings and nm-remote-settings-system, the
removal of user settings services means there is no more need for
separate interfaces for user and system settings services.

In libnm-glib, this commit merges everything in
nm-settings-system-interface into nm-settings-interface.  Alongside with
that, we merge everything in the
org.freedesktop.NetworkManagerSettings.System DBus interface into
org.freedesktop.NetworkManagerSettings.