[ironic] Deprecate/Remove PowerPC prep partition support?

Greetings folks, I've been curious if anyone sees a reason to keep PowerPC PReP partition support in Ironic? A few things to consider: * This is/was to support partition images. * We generally shy away from partition images at this point. We recommend whole disk images. * PowerPC.... And I don't think many (any?) operators will be doing power deployments of Z* and beyond OpenStack once it is released. * This is not tested in CI, because Power hardware... Thoughts? Feelings? Concerns? -Julia