remove channel group model

* Websocket user session <-> group subscription membership now resides
in Redis rather than the database.

awx/main/migrations/0110_delete_channelgroup.py

@@ -0,0 +1,16 @@
+# Generated by Django 2.2.8 on 2020-02-17 14:50
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('main', '0109_v370_instance_ip_address'),
+    ]
+
+    operations = [
+        migrations.DeleteModel(
+            name='ChannelGroup',
+        ),
+    ]