Clean usage of project_id in the Neutron's L3 service module

After patch [1] project_id in that module is no longer needed as to make
it working with new secure RBAC policies we had to hardcode "demo"
project to be used always.
This is small follow-up patch with cleaning after [1].

[1] https://review.opendev.org/c/openstack/devstack/+/826851/

Change-Id: Iddf9692817c91807fc3269547910e4f83585f07f
1 file changed