AS - ASContentModel - constant AS_UNDEFINED not necessary

Dear all!

I propose that the constant AS_UNDEFINED is dropped as a list operator for
ASContentModel.

This constant should be used only intermediatly by AS implementation for
content models that are created but not yet finished.
Such intermediate states may be necessary (because of recursive data
structures). However, this is an implementation detail that is completely in the
responsibility of an AS implementation provider.

Thanks for your attention,
--Stefan