code: def unregister_event(self, event_name, handler):
          
          try:
              self._event_handlers[event_name].remove(handler)
          except ValueError:
              pass
docs: Unregister a callable that will be called when event is raised.
  
          :param event_name: The name of the event (see knack.events for in-built events)
          :type event_name: str
          :param handler: The callback that was used to register the event
          :type handler: function