MDL-43511 do not cache the MUC cache in question code

question/engine/bank.php

@@ -435,9 +435,6 @@ class question_finder implements cache_data_source {
     /** @var question_finder the singleton instance of this class. */
     protected static $questionfinder = null;
 
-    /** @var cache the question definition cache. */
-    protected $cache = null;
-
     /**
      * @return question_finder a question finder.
      */
@@ -457,10 +454,8 @@ public static function get_instance_for_cache(cache_definition $definition) {
      * @return get the question definition cache we are using.
      */
     protected function get_data_cache() {
-        if ($this->cache == null) {
-            $this->cache = cache::make('core', 'questiondata');
-        }
-        return $this->cache;
+        // Do not double cache here because it may break cache resetting.
+        return cache::make('core', 'questiondata');
     }
 
     /**