dev/core#2073 Fix use of legacy leaky method in tested code

This is too low volume to really leak but it uses the leaky legacy method and
has test cover in

CRM_Contact_Import_Form_MapFieldTest.testSubmit with data set basic_data
CRM_Contact_Import_Form_MapFieldTest.testSubmit with data set save_mapping

CRM/Contact/Import/Form/MapField.php

@@ -149,12 +149,9 @@ public function preProcess() {
     }
     else {
       // get the field names from the temp. DB table
-      $dao = new CRM_Core_DAO();
-      $db = $dao->getDatabaseConnection();
-
       $columnsQuery = "SHOW FIELDS FROM $this->_importTableName
                          WHERE Field NOT LIKE '\_%'";
-      $columnsResult = $db->query($columnsQuery);
+      $columnsResult = CRM_Core_DAO::executeQuery($columnsQuery);
       while ($row = $columnsResult->fetchRow(DB_FETCHMODE_ASSOC)) {
         $columnNames[] = $row['Field'];
       }