It's unused by any of our nsISAXXMLReader implementations.
The patch also removes nsSAXXMLReader::mEnableNamespacePrefixes, which is now
unused. And it removes test_namespace_support.js, which is all about testing
features.
Because none of our nsISAXXMLReader implementations set `dtdHandler`,
`declarationHandler`, or `lexicalHandler`.
The patch also removes test_xml_declaration.js, because that test is all about
testing nsIMozSAXXMLDeclarationHandler.