setup-rust: remove the legacy 'direct-ffi' variant
This variant have been abandoned for a while. Keeping it around just get people
confused.
#include <Python.h>
#if PY_MAJOR_VERSION >= 3
#define PYCODETYPE PyObject
#else
#define PYCODETYPE PyCodeObject
#endif
namespace contrib
{
void initpy(const char *cselfpath);
PyObject *pyglobals();
} /* namespace contrib */