get rid of cubicweb_*.spec files?

Apparently this is only used for client projects that uses redhat/fedora but we aren't sure that there are any that do that anymore. In addition, if everyone move to docker we won't need it anymore.

This is, apparently, still present in a cube skeleton.

cc @nchauvat

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information