remove _append_null_domain_id decorator
remove _append_null_domain_id decorator

This decorator is only useful when upgrade Keystone form Liberty
to higher release. I don't think any deployments will upgrade
from Liberty to Queens directly. And Liberty is eol already. So
it's safe engouth to remove it.

Change-Id: I891b0d3e87e8c011a2db758fc84dbd4590f78c96

openstack/keystone,openstack/keystone,mahak/keystone,openstack/keystone,mahak/keystone,mahak/keystone