install: drop RHEL/CentOS 7 doc
install: drop RHEL/CentOS 7 doc

the packages there are too outdated to build properly latest versions
of buildah, e.g. a different go toolchain must be installed manually
as well as gpgme-devel.

Signed-off-by: Giuseppe Scrivano <20002b7b24c75512093a71a58285826d5c504fca@redhat.com>

TomSweeneyRedHat/buildah,rhatdan/buildah,rhatdan/buildah,rhatdan/buildah,TomSweeneyRedHat/buildah,nalind/buildah-1,TomSweeneyRedHat/buildah,nalind/buildah-1,nalind/buildah-1