[1.11.x] Removed unused imports in example migrations.

Backport of a76d12ceb4 from master

docs/ref/migration-operations.txt

@@ -317,7 +317,7 @@ model::
     # -*- coding: utf-8 -*-
     from __future__ import unicode_literals
 
-    from django.db import migrations, models
+    from django.db import migrations
 
     def forwards_func(apps, schema_editor):
         # We get the model from the versioned app registry;