Author: Simey Lameze
MDL-65050 core: add question_stats_cleanup_task scheduled task
This commit also removes orphaned question/engine/statisticslib.php
http://git.moodle.org/gw?p=moodle.git;a=commit;h=9c164d26fc9f62fae8eae10f4a36e80f8bc70e16
http://github.com/moodle/moodle/commit/9c164d26fc9f62fae8eae10f4a36e80f8bc70e16
+ 3.7 en [taskquestionstatscleanupcron,core_admin]