[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):

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;

docs/topics/migrations.txt

@@ -463,7 +463,7 @@ Then, open up the file; it should look something like this::
     # 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
 
     class Migration(migrations.Migration):
         initial = True
@@ -493,7 +493,7 @@ need to do is use the historical model and iterate over the rows::
     # -*- coding: utf-8 -*-
     from __future__ import unicode_literals
 
-    from django.db import migrations, models
+    from django.db import migrations
 
     def combine_names(apps, schema_editor):
         # We can't import the Person model directly as it may be a newer