olso.policy 4.5.0[1] changed the config options policy_file
default value to 'policy.yaml', which means it is changed
for all the OpenStack services and they do not need to
override the default anymore.

NOTE: There is no change in behaviour here, oslo.policy provides
the same configuration that services have overridden till now.

[1] https://review.opendev.org/c/openstack/releases/+/934012
[2] https://review.opendev.org/c/openstack/requirements/+/934295

Change-Id: Iaa414ee88325bd0aa8c6c135baf1712dd6b3c6a7