CRM-12998 remove add to group from email summary report

CRM/Report/Form/Mailing/Summary.php

@@ -42,6 +42,8 @@ class CRM_Report_Form_Mailing_Summary extends CRM_Report_Form {
 
   protected $_customGroupExtends = array();
 
+  protected $_add2groupSupported = FALSE;
+
   public $_drilldownReport = array('contact/detail' => 'Link to Detail Report');
 
   protected $_charts = array(