MDL-75576 quiz/question statistics: don't expire by time

Previously, a set of calculated quiz statistics would only 'last' for
15 minutes. Then they would be considered invalid and not used.

Now, computed statistics are kept indefinitely. Instead, when a new
batch of values are computed for a particular set of settings, older numbers
for the same settings are deleted first. Therefore,
question_stats_cleanup_task is no more.

lib/classes/task/question_stats_cleanup_task.php

@@ -1,65 +0,0 @@
-<?php
-// This file is part of Moodle - http://moodle.org/
-//
-// Moodle is free software: you can redistribute it and/or modify
-// it under the terms of the GNU General Public License as published by
-// the Free Software Foundation, either version 3 of the License, or
-// (at your option) any later version.
-//
-// Moodle is distributed in the hope that it will be useful,
-// but WITHOUT ANY WARRANTY; without even the implied warranty of
-// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-// GNU General Public License for more details.
-//
-// You should have received a copy of the GNU General Public License
-// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
-
-/**
- * Task to cleanup old question statistics cache.
- *
- * @package    core
- * @copyright  2019 Simey Lameze <[email protected]>
- * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
- */
-namespace core\task;
-
-defined('MOODLE_INTERNAL') || die();
-
-/**
- * A task to cleanup old question statistics cache.
- *
- * @copyright  2019 Simey Lameze <[email protected]>
- * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
- */
-class question_stats_cleanup_task extends scheduled_task {
-
-    /**
-     * Get a descriptive name for this task (shown to admins).
-     *
-     * @return string
-     */
-    public function get_name() {
-        return get_string('taskquestionstatscleanupcron', 'admin');
-    }
-
-    /**
-     * Perform the cleanup task.
-     */
-    public function execute() {
-        global $DB;
-
-        mtrace("\n  Cleaning up old question statistics cache records...", '');
-
-        $expiretime = time() - 5 * HOURSECS;
-        $DB->delete_records_select('question_statistics', 'timemodified < ?', [$expiretime]);
-        $responseanlysisids = $DB->get_records_select_menu('question_response_analysis',
-            'timemodified < ?',
-            [$expiretime],
-            'id',
-            'id, id AS id2');
-        $DB->delete_records_list('question_response_analysis', 'id', $responseanlysisids);
-        $DB->delete_records_list('question_response_count', 'analysisid', $responseanlysisids);
-
-        mtrace('done.');
-    }
-}