Drop the JAXB to JSON marshalling tests

optaplanner-persistence-jaxb supports score serialization also to JSON, if an alternative JAXB implementation (Moxy) is used.

The regression tests of this feature are difficult to migrate to Jakarta due to https://github.com/openrewrite/rewrite/issues/2603, and, more importantly, are redundant. It's Moxy's responsibility to make sure it correctly serializes the JAXB-annotated domain to JSON.