These jobs started to suddenly fail inexplicably. They also
require privileged mode and are a mess in general

Build and packaging for these distros is handled by nightly OBS builds
nowadays, so let's remove these.

Perhaps in the future, it might make sense ot bring back some sensible
CentOS 7 build (without the packaging part) to make sure we keep
compatibility with older GnuTLS on every commit / MR, instead of
nightly.