`core` remove unused `cloudEvents` serializer and deserializer and hence dependency

[francoisledroff]

• It was introduced here Add support for Registration V2 API #126

  • not sure why we introduced it,
  • ISO Date formatting is coming for free in Jackson when using @JsonFormat

• we still have this events_ingress : replace our CloudEvent model by the Open source cloudevents/sdk-java model . #16 pending

• but for now I propose we remove this useless dependency