remove nif:topic

[VladimirAlexiev]

@der-bruemmer says in #10: "The point of this property was to annotate a complete text with topical tags or entity classes, as opposed to a single entity... I'll ask the Gerbillians for more information."

These questions of mine were not answered, and more information from Gerbilians was not received:

• If it's a property of the complete text, why not define the domain as nif:Context?
• For "topical tag", why not use dct:subject but invent a new property?
  (Actually, its:taIdentRef is better for this purpose)
• For "entity class" why not use its:taClassRef but invent a new property?

I don't see a need for such property, we can use its:taIdentRef or its:taClassRef