MDL-30514 registration: table aliases don't use AS keyword

This is a backport of both MDL-34072, MDL-34440 (initial fix
and following regression). Basically it's about to observe #12 of:
    http://docs.moodle.org/dev/Database
(Use the AS keyword always with column aliases but not with table aliases)

course/lib.php

@@ -3945,7 +3945,7 @@ function average_number_of_participants() {
         WHERE ue.enrolid = e.id
             AND e.courseid <> :siteid
             AND c.id = e.courseid
-            AND c.visible = 1) as total';
+            AND c.visible = 1) total';
     $params = array('siteid' => $SITE->id);
     $enrolmenttotal = $DB->count_records_sql($sql, $params);
 
@@ -3978,7 +3978,7 @@ function average_number_of_courses_modules() {
         WHERE c.id = cm.course
             AND c.id <> :siteid
             AND cm.visible = 1
-            AND c.visible = 1) as total';
+            AND c.visible = 1) total';
     $params = array('siteid' => $SITE->id);
     $moduletotal = $DB->count_records_sql($sql, $params);