Remove about_node_representation

```json
{
  "id": "CHANGE:001",
  "old_value": "'foo'",
  "new_value": "'bar'",
  "about_node": "GO:1",
  "about_node_representation": "curie",
  "@type": "NodeRename"
} 
```

This is unneccessary - just as with jsonld we can leave as either CURIE or URI. Of course, wwe should always have a context file that maps prefixes