Archive/Remove this project

I've been told were basically tracking upstream for clang. Is this true? Then we could archive or remove this project.

TODO (in case you decide to remove/archive):

• Adapt https://gitlab.dune-project.org/joe/pacxx-docker to use upstream directly
• Adapt https://zivgitlab.uni-muenster.de/HPC2SE-Project/pacxx-runtime to use upstream directly