MDL-30509 backport perf improvement in role cleanup in cohort sync

This patch simplifies the role cleanup sql, it is not necessary to look for roles without enrolment instance because all roles are automatically unassigned when deleting enrol instances.

enrol/cohort/locallib.php

@@ -173,15 +173,13 @@ function enrol_cohort_sync($courseid = NULL) {
     }
 
     // remove unwanted roles - include ignored roles and disabled plugins too
-    $onecourse = $courseid ? "AND c.instanceid = :courseid" : "";
+    $onecourse = $courseid ? "AND e.courseid = :courseid" : "";
     $sql = "SELECT ra.roleid, ra.userid, ra.contextid, ra.itemid
               FROM {role_assignments} ra
-              JOIN {context} c ON (c.id = ra.contextid AND c.contextlevel = :coursecontext $onecourse)
-         LEFT JOIN (SELECT e.id AS enrolid, e.roleid, ue.userid
-                      FROM {user_enrolments} ue
-                      JOIN {enrol} e ON (e.id = ue.enrolid AND e.enrol = 'cohort')
-                   ) x ON (x.enrolid = ra.itemid AND ra.component = 'enrol_cohort' AND x.roleid = ra.roleid AND x.userid = ra.userid)
-             WHERE x.userid IS NULL AND ra.component = 'enrol_cohort'";
+              JOIN {context} c ON (c.id = ra.contextid AND c.contextlevel = :coursecontext)
+              JOIN {enrol} e ON (e.id = ra.itemid AND e.enrol = 'cohort' $onecourse)
+         LEFT JOIN {user_enrolments} ue ON (ue.enrolid = e.id AND ue.userid = ra.userid)
+             WHERE ra.component = 'enrol_cohort' AND ue.id IS NULL";
     $params = array('coursecontext' => CONTEXT_COURSE, 'courseid' => $courseid);
 
     $rs = $DB->get_recordset_sql($sql, $params);