unnecessary checks in assignment/controllers.py

[wanghong]

There are checks that only one of project/domain or user/group are passed to create/list/check/revoke_grant methods, https://github.com/openstack/keystone/blob/master/keystone/assignment/controllers.py#L496-L504. However,
1. only router calls these methods
2. URL scheme does not accept both project and user simultaneously.
If a user specify a url such as /domains/{domain_id}/projects/{project_id}/users/{user_id}/roles or /domains/{domain_id}/users/{user_id}/groups/{group_id}/roles or etc, he would get a 404 error from router.
That is to say, the cases these methods check would never happen. So, I think these checks can be removed.

[OpenStack Infra (hudson-openstack)]

Change abandoned by wanghong (<email address hidden>) on branch: master
Review: https://review.openstack.org/130722

[Boris Bobrov (bbobrov)]

Setting to invalid because the patch is abandoned and because of comments in it.