Remove invalid id attribute from DFC offer

This one is a bit confusing. The attribute changes throughout the
different versions of the ontology and the DFC examples are not in line
with the specification. WIP I guess.

The previous nil attribute failed JSON-LD validation and DFC examples
just contain the reference as I did it now. But I think that we will
need to update it again vor DFC-v1.7.

engines/dfc_provider/app/serializers/dfc_provider/offer_serializer.rb

@@ -20,8 +20,7 @@ module DfcProvider
 
     def offers_to
       {
-        '@type' => '@id',
-        '@id' => nil
+        '@type' => '@id'
       }
     end