Converting EER diagram to ER diagram?
The only way I know of is one I can't recommend.  

If you convert your EERD into a relational design (with tables, etc.) using some technique like "class table inheritance", you have now disguised the fact that the original EERD contained a generalization/specialization pattern.

If you now turn around and reverse engineer that back to an ERD, you have something that satisfies the requirements of your question.  But that's grotesque.  It conceals the original intent rather than revealing it.  Why would anyone ever want to do that?