Remove RDF search - incompatible with structured JSON search

[joepio]

In the earlier iterations of Atomic Data, we indexed triples in the search engine. However, since tantivy supports indexing full JSON objects, we changed the logic. Now I think the RDF search code is becoming legacy, and should be removed. Let me know if anyone is actually using this - maybe we could create a new rust project that specializes in RDF search, based on the code that I'm about to remove.