[cinder] removing block-box code

Hello Argonauts and other interested parties, As you may be aware, the cinder code repository contains files under contrib/block-box whose README file has included a note [0] since 20 January 2020 indicating that block-box is not supported. This note serves notice that we will be removing the code during the Yoga development cycle. I trust that this will be uncontroversial. If you have any objections, please raise them now, either here on the ML or on the patch removing the code: https://review.opendev.org/c/openstack/cinder/+/824472 cheers, brian [0] https://opendev.org/openstack/cinder/commit/a4d8f761639d63172814a593a32da416...