Remove one of `empty_typet` or `void_typet`

[tautschnig]

Those are defined the same way, meaning empty_typet() == void_typet() actually evaluates to true.

[NlightNFotis]

Could it be that the removal of this might hinder support for other languages in the future? Is it possible that we might want to support a language that needs support for both of these?

[tautschnig]

void_typet has been removed in #4102.