Drop support unqualified-search-registry for artifacts

Artifact implementation in containers/container-libs doesn't support unqualified search registry.
https://github.com/containers/container-libs/issues/406

Since we already set shortnames enforced for images and artifacts by default https://github.com/cri-o/cri-o/pull/9401 , I think we should drop support of unqualified search registry for artifacts too.

wdyt? @cri-o/cri-o-maintainers 