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