Don't identify primary topic in external JSON-LD, only internally

Identifying the primary topic was originally both for the request in https://github.com/lobid/lodmill/issues/230#issuecomment-32290484 and for #257. It did break Edoweb however, probably due to an issue with parsing named JSON-LD graphs in rdflib. This removes the primary topic from the served JSON-LD, but retains it in the internal format, used by the UI for #257.