code: function deleteListener(event, callback, idx) {
    if ((idx = event.listeners.indexOf(callback)) >= 0) {
      event.listeners.splice(idx, 1);
      event.meta.delete(callback);
      return true;
    }
    return false;
  }
docs: Delete event listener returns true on success
  
  @param {Object} event
  @param {Function} callback
  @param {number} [idx]
  @returns {boolean}