Remove the RDF module as a dependency

The RDF module was moved to contrib because not many sites use it, because the added value is low. The current move is towards json-ld and structured data with that. We now require the rdf module and have it as a dependency for the SEO module. Removing it as a dependency is a small step to make rocketship a bit leaner without losing any important functionality. This was checked with our SEO experts,

I'd propose to remove it from the seo module in the info.yml file (and if necessary, other info.yml files) and also from the composer.json and provide instructions it needs to be added to the root composer.json of a project if it needs to be kept.