Remove default override for config options policy_file

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: I46cc9e05fbc8f6c95c0b2d50093ecfb070a4170f

requirements.txt

@@ -18,7 +18,7 @@ oslo.db>=4.44.0 # Apache-2.0
 oslo.i18n>=3.20.0 # Apache-2.0
 oslo.log>=3.37.0 # Apache-2.0
 oslo.messaging>=14.1.0 # Apache-2.0
-oslo.policy>=3.6.0 # Apache-2.0
+oslo.policy>=4.5.0 # Apache-2.0
 oslo.reports>=1.27.0 # Apache-2.0
 oslo.serialization>=2.25.0 # Apache-2.0
 oslo.service>=1.30.0 # Apache-2.0