def purge(request):
    """
        Called when a node purge request has been received.

        @param request: The publish-subscribe request.
        @type request: L{wokkel.pubsub.PubSubRequest}
        @return: A deferred that fires with C{None} when the node has been
                 purged.
        @rtype: L{Deferred<twisted.internet.defer.Deferred>}
        """
