ValueOnly: do not mention types omitted

[BirgitBoss]

Is your feature request related to a problem? Please describe. There is a statement

`The following types defined by the XSD and RDF specifications are explicitly omitted for serialization:

xs:language, xs:normalizedString, xs:token, xs:NMTOKEN, xs:Name, xs:NCName, xs:QName, xs:ENTITY, xs:ID, xs:IDREF, xs:NOTATION, xs:IDREFS, xs:ENTITIES, xs:NMTOKENS, rdf:HTML and rdf:XMLLiteral.`

However, these types are not allowed in the first place so this does not need to be repeated for DataTypeDefXsd