correctly cascade set null

* It's problematic to delete an instance that is referenced by a foreign
key; where the referening model is one that has a Polymorphic parent.
* Specifically, when Django goes to nullify the relationship it relies
on the related instances[0] class type to issue a query to decide what
to nullify. So if the foreignkey references multiple different types
(i.e. ProjectUpdate, Job) then only 1 of those class types will get
nullified. The end result is an IntegrityError when delete() is called.
* This changeset ensures that the parent Polymorphic class is queried so
that all the foreignkey entries are nullified
* Also remove old Django "hack" that doesn't work with Django 1.11

awx/main/migrations/0030_v330_polymorphic_delete.py

@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.11 on 2018-04-06 13:44
+from __future__ import unicode_literals
+
+import awx.main.utils.polymorphic
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('main', '0029_v330_encrypt_oauth2_secret'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='unifiedjob',
+            name='instance_group',
+            field=models.ForeignKey(blank=True, default=None, help_text='The Rampart/Instance group the job was run under', null=True, on_delete=awx.main.utils.polymorphic.SET_NULL, to='main.InstanceGroup'),
+        ),
+    ]