Remove unique constraint for proxy class field of offer condition model.

src/oscar/apps/offer/abstract_models.py

         _('Value'), decimal_places=2, max_digits=12, null=True, blank=True)
 
     proxy_class = fields.NullCharField(
-        _("Custom class"), max_length=255, unique=True, default=None)
+        _("Custom class"), max_length=255, default=None)
 
     class Meta:
         abstract = True

src/oscar/apps/offer/migrations/0003_auto_20161120_1707.py

+# -*- coding: utf-8 -*-
+# Generated by Django 1.9.11 on 2016-11-20 17:07
+from __future__ import unicode_literals
+
+from django.db import migrations
+import oscar.models.fields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('offer', '0002_auto_20151210_1053'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='condition',
+            name='proxy_class',
+            field=oscar.models.fields.NullCharField(default=None, max_length=255, verbose_name='Custom class'),
+        ),
+    ]