session: get rid of the worker_connection state variable in conversation object
    
    The conversation object maintains a pointer to the worker connection and
    to the worker proxy.  The proxy already keeps a reference on the worker
    connection, so there's no need to keep an explicit reference on the
    connection directly.
    
    This commit drops this extra reference.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=727589