Undo the polymorphic.SET_NULL for Organization

It isn't polymorphic.

awx/main/migrations/0131_undo_org_polymorphic_ee.py

@@ -0,0 +1,19 @@
+# Generated by Django 2.2.16 on 2021-03-11 20:50
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('main', '0130_ee_polymorphic_set_null'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='organization',
+            name='default_environment',
+            field=models.ForeignKey(blank=True, default=None, help_text='The default execution environment for jobs run by this organization.', null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='+', to='main.ExecutionEnvironment'),
+        ),
+    ]

awx/main/models/organization.py

@@ -27,7 +27,6 @@ from awx.main.models.rbac import (
 )
 from awx.main.models.unified_jobs import UnifiedJob
 from awx.main.models.mixins import ResourceMixin, CustomVirtualEnvMixin, RelatedJobsMixin
-from awx.main.utils import polymorphic
 
 __all__ = ['Organization', 'Team', 'Profile', 'UserSessionMembership']
 
@@ -67,7 +66,7 @@ class Organization(CommonModel, NotificationFieldsModel, ResourceMixin, CustomVi
         null=True,
         blank=True,
         default=None,
-        on_delete=polymorphic.SET_NULL,
+        on_delete=models.SET_NULL,
         related_name='+',
         help_text=_('The default execution environment for jobs run by this organization.'),
     )