MDL-65050 core: add question_stats_cleanup_task scheduled task

This commit also removes orphaned question/engine/statisticslib.php

lib/db/tasks.php

@@ -221,6 +221,15 @@ $tasks = array(
         'dayofweek' => '*',
         'month' => '*'
     ),
+    array(
+        'classname' => 'core\task\question_stats_cleanup_task',
+        'blocking' => 0,
+        'minute' => '*',
+        'hour' => '*',
+        'day' => '*',
+        'dayofweek' => '*',
+        'month' => '*'
+    ),
     array(
         'classname' => 'core\task\registration_cron_task',
         'blocking' => 0,