Remove unnecessary revocation events
Remove unnecessary revocation events

With [0], we no longer need role_assignment callback. It isn't being
used anywhere and token providers rebuild tokens at validation time.
Some links to show test coverage for role_assignment:
https://github.com/openstack/keystone/blob/master/keystone/tests/unit/test_auth.py#L526-L553
https://github.com/openstack/keystone/blob/master/keystone/tests/unit/test_v3_auth.py#L2993
https://github.com/openstack/keystone/blob/master/keystone/tests/unit/test_v3_auth.py#L3179
https://github.com/openstack/keystone/blob/master/keystone/tests/unit/test_v3_auth.py#L3329
https://github.com/openstack/keystone/blob/master/keystone/tests/unit/test_v3_auth.py#L3610
https://github.com/openstack/keystone/blob/master/keystone/tests/unit/test_v3_assignment.py#L31-L33

[0]: https://review.openstack.org/#/c/447562/

Change-Id: If690c8ff8466e8568a8c9dc4463f341adb675630
partial-bug: 1671887

ilay09/keystone,openstack/keystone,rajalokan/keystone,openstack/keystone,ilay09/keystone,ilay09/keystone,openstack/keystone,mahak/keystone,rajalokan/keystone,rajalokan/keystone,mahak/keystone,mahak/keystone