MDL-35252: Fix a bug that creates unneccessary rows in grade_grades t…

…able when unchecking 'overridden' check box in gradebook update

mod/assign/locallib.php

@@ -4341,7 +4341,7 @@ public function get_user_grades_for_gradebook($userid) {
         $graderesults = $DB->get_recordset_sql('SELECT u.id as userid, s.timemodified as datesubmitted, g.grade as rawgrade, g.timemodified as dategraded, g.grader as usermodified
                             FROM {user} u
                             LEFT JOIN {assign_submission} s ON u.id = s.userid and s.assignment = ?
-                            LEFT JOIN {assign_grades} g ON u.id = g.userid and g.assignment = ?
+                            JOIN {assign_grades} g ON u.id = g.userid and g.assignment = ?
                             ' . $where, array($assignmentid, $assignmentid, $userid));