Remove ExternalPojo Interface and ExternallyMarshallable

ExternallyMarshallable & ExternalPojo don't really make sense with the latest marshaller changes (ISPN-10354), as the Global & Persistence marshallers can only handle types they know about. Unknown types are always handled by the user marshaller and when JavaSerializationMarshaller is used (the default now) the user has to explicitly add classes to the white list.