The following has been borrowed from [here][1]. 

Insert the following into your custom module.

    /**
     * Implements hook_node_delete().
     */
    function mymodule_node_delete($node) {
      if ($node->type == 'mynodetype'){
        $nids = db_query('SELECT entity_id FROM {field_data_field_review_reference} WHERE field_review_reference_nid = :nid', array(':nid' => $node->nid))->fetchCol();
        node_delete_multiple($nids);
      }
    }


  [1]: https://drupal.stackexchange.com/questions/8054/how-to-manage-and-delete-orphaned-node-references