Eventually, I will refactor libnm to no longer use gdbus-codegen and
no GDBusProxy. In preparation of that, we must stop using that
API.

As first step, change nm_client_deactivate_connection(). Note how this
was done previously:

 - nm_client_deactivate_connection() calls nm_manager_deactivate_connection()
   - nmdbus_manager_call_deactivate_connection_sync() calls g_dbus_proxy_call_sync()
     - g_dbus_proxy_call_sync() calls g_dbus_connection_call_sync()

Currently this is still a bit ugly, because NMClient doesn't directly
track the GDBusConnection nor the name owner. Instead, we need to peel
it out of the object manager. One day, that will all be nicer, but first
get rid of gdbus-codegen.