Unregisters any registered flow processing handler.

def UnregisterFlowProcessingHandler(self, timeout=None):
    """Unregisters any registered flow processing handler."""
    if self.flow_processing_request_handler_thread:
      self.flow_processing_request_handler_stop = True
      self.flow_processing_request_handler_thread.join(timeout)
      if self.flow_processing_request_handler_thread.isAlive():
        raise RuntimeError("Flow processing handler did not join in time.")
      self.flow_processing_request_handler_thread = None