Remove unused code in _make_posargs

`names_with_default` is never `NULL`, even if there are no names with defaults. In that case it points to a structure with `size` zero.

Author: mjdominus

Parser/action_helpers.c

@@ -554,9 +554,6 @@ _make_posargs(Parser *p,
  else if (plain_names == NULL && names_with_default != NULL) {
  *posargs = _get_names(p, names_with_default);
  }
- else if (plain_names != NULL && names_with_default == NULL) {
- *posargs = plain_names;
- }
  else {
  *posargs = _Py_asdl_arg_seq_new(0, p->arena);
  }