Replace dump_ct::make_decl once #2713 is merged

[tautschnig]

With #2713 there is a variant of type2c that build a declaration string, making the hack in dump_ct::make_decl no longer necessary.

[tautschnig]

make_decl actually is a useful utility, despite any improvements to type2c. There are a bunch of places that could surely be cleaned up in dump_ct, but this is much more general than what could be gained from #2713. Closing for the time being.