Resolve "mvv_assignments bzw. mvv_contacts kann nicht nach priority sortiert werden"

Closes #1825

Merge request studip/studip!1200

lib/models/User.class.php

@@ -169,8 +169,7 @@ class User extends AuthUserMd5 implements Range, PrivacyObject
         $config['has_many']['mvv_assignments'] = [
             'class_name'        => MvvContact::class,
             'assoc_foreign_key' => 'contact_id',
-            'on_delete'         => 'delete',
-            'order_by'          => 'ORDER BY priority',
+            'on_delete'         => 'delete'
         ];
 
         $config['has_and_belongs_to_many']['domains'] = [