MDL-67312 core: change db upgrade step to use get_fieldset_sql

lib/db/upgrade.php

@@ -1845,7 +1845,7 @@ function xmldb_main_upgrade($oldversion) {
                   FROM {event_subscriptions} es
              LEFT JOIN {user} u ON u.id = es.userid
                  WHERE u.deleted = 1 OR u.id IS NULL";
-        $deletedusers = $DB->get_field_sql($sql);
+        $deletedusers = $DB->get_fieldset_sql($sql);
         if ($deletedusers) {
             list($sql, $params) = $DB->get_in_or_equal($deletedusers);