code: @Override
      public void unregisterAttributeListener(String attributeName, AttributeListener attributeListener) {
          List<AttributeListener> listeners = attributeListeners.get(attributeName);
          if (listeners == null) {
              LOG.error("trying to unregister an attribute listener for attribute \"" + attributeName
                      + "\" that was never registered");
              return;
          }
          synchronized (listeners) {
              boolean success = listeners.remove(attributeListener);
              if (!success) {
                  LOG.error("trying to unregister an attribute listener for attribute \"" + attributeName
                          + "\" that was never registered");
                  return;
              }
          }
      }
docs: Unregisters an attribute listener.
  
  @param attributeName     the attribute name as defined in the Franca model
  to unsubscribe from.
  @param attributeListener the listener to remove.