bpo-42000: Cleanup the AST related C-code (GH-22641)

- Use the proper asdl sequence when creating empty arguments
- Remove reduntant casts (thanks to new typed asdl_sequences)
- Remove MarshalPrototypeVisitor and some utilities from asdl generator
- Fix the header of `Python/ast.c` (kept from pgen times)

Automerge-Triggered-By: @pablogsal