Remove obsolete creation_counter field
Remove obsolete creation_counter field

creation_counter is never used in fields.py and was deprecated in
Django 2.x. Fields of object are keeped internally as ordered
dictionary, so we don't need any additional tools to manage it

Related Django bug:
https://code.djangoproject.com/ticket/23919#comment:95

Change-Id: I530a47ad8bfed6aef63d0a678a69fa09288f123e

NeCTAR-RC/horizon,ChameleonCloud/horizon,openstack/horizon,ChameleonCloud/horizon,NeCTAR-RC/horizon,ChameleonCloud/horizon,NeCTAR-RC/horizon,openstack/horizon,NeCTAR-RC/horizon,openstack/horizon,ChameleonCloud/horizon,openstack/horizon