Remove unused import

Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/migrations/0013_auto_20141124_1036.py

 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
-from django.db import models, migrations
+from django.db import migrations
 from django.db.models.aggregates import Sum
 
 from weblate.trans.boolean_sum import BooleanSum