Special:EntityData currently supports various RDF serializations, as well as API style JSON and XML. Dropping support for the API style XML would allow us to remove the awkward dependency and ugly code caused by abusing the API classes for generating the output of a special page.
Instead, we would generate the JSON output directly, like we do for JSON dumps and DB storage.
Note that the API style XML format is not documented for Wikibase, it's basically an artifact how how the MediaWiki API works. It's use is discouraged even for the API, and I see no reason we should expose it via our linked data interface.