Since 7.10, the JSON (un)marshalling is done by nuxeo-core-io, so it's time to remove this legacy message body reader.
DocumentModelJsonReaderLegacy needs also to be removed as it's the Reader plugging the legacy message body reader in the nuxeo-core-io (un)marshalling.
Update documentation to remove references to:
- the system property nuxeo.document.json.legacy
- the header X-NXDocumentJsonLegacy
Both are only used by DocumentModelJsonReaderLegacy.