MDL-42664 Fix the filter_manager instantiation

For legacy reasons, the $CFG->perfdebug uses values 7 for disabled and 15 for
enabled. Because of this typo, Moodle has always created instance of the slower
performance_measuring_filter_manager instead of the standard filter_manager,
regardless the perfdebug setting.

lib/filterlib.php

@@ -81,7 +81,7 @@ class filter_manager {
     public static function instance() {
         global $CFG;
         if (is_null(self::$singletoninstance)) {
-            if (!empty($CFG->perfdebug)) {
+            if (!empty($CFG->perfdebug) and $CFG->perfdebug > 7) {
                 self::$singletoninstance = new performance_measuring_filter_manager();
             } else {
                 self::$singletoninstance = new self();