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

Backport of a76d12ceb4 from master

docs/howto/writing-migrations.txt

@@ -158,7 +158,7 @@ the respective field according to your needs.
     # Generated by Django A.B on YYYY-MM-DD HH:MM
     from __future__ import unicode_literals
 
-    from django.db import migrations, models
+    from django.db import migrations
     import uuid
 
     def gen_uuid(apps, schema_editor):