migrations: Remove unused imports.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

pgroonga/migrations/0001_enable.py

@@ -1,6 +1,5 @@
 # -*- coding: utf-8 -*-
-from django.db import models, migrations
-from django.contrib.postgres import operations
+from django.db import migrations
 from django.conf import settings
 
 

pgroonga/migrations/0002_html_escape_subject.py

@@ -1,6 +1,5 @@
 # -*- coding: utf-8 -*-
-from django.db import models, migrations, connection
-from django.contrib.postgres import operations
+from django.db import migrations, connection
 from django.db.backends.postgresql_psycopg2.schema import DatabaseSchemaEditor
 from django.db.migrations.state import StateApps
 from zerver.lib.migrate import do_batch_update

pgroonga/migrations/0003_v2_api_upgrade.py

@@ -1,6 +1,5 @@
 # -*- coding: utf-8 -*-
-from django.db import models, migrations
-from django.contrib.postgres import operations
+from django.db import migrations
 from django.conf import settings