ExtensionTypeCustomDeserializers's addTargetClass() and addTargetTypeReference() wrongly expect `data` filed in ext type value is MessagePack format

[komamitsu]

The 2 methods wrongly expect data field in ext type value is MessagePack format, but the format of ext type data field (https://github.com/msgpack/msgpack/blob/master/spec.md#ext-format-family) isn't supposed to be MessagePack. So the 2 methods should be removed.