async notifyDocumentDelete(request, document) {
    const rooms = this._test(request, document._source, document._id);

    if (rooms.length > 0) {
      await this.notifyDocument(rooms, request, "out", "delete", document);
    }

    return [];
  }
Notify about a document deletion

@param {Request} request
@param {DocumentChanges} document
@returns {Promise.<Array>} returns an empty array ("no room match anymore")