code: public MapUpdate removeSet(String key)
      {
          BinaryValue k = BinaryValue.create(key);
          removes.add(new MapOp.MapField(MapOp.FieldType.SET, k));
          return this;
      }
docs: Update the map in Riak by removing the set mapped to the provided key.
  @param key the key the set is mapped to.
  @return a reference to this object.