Cover DELETEing the license from database config

Fix the standard license delete behavior on the /config endpoint.  It
was covered elsewhere but this is the common path for license posting/removing

awx/api/views.py

@@ -290,6 +290,8 @@ class ApiV1ConfigView(APIView):
                     has_error = e.errno
                     break
 
+        TowerSettings.objects.filter(key="LICENSE").delete()
+
         # Only stop mongod if license removal succeeded
         if has_error is None:
             mongodb_control.delay('stop')