Removed unused internal C API functions

The functions in Include/internal/pycore_identifier.h are not used:

```
extern PyObject* _PyType_LookupId(PyTypeObject *, _Py_Identifier *);
extern PyObject* _PyObject_LookupSpecialId(PyObject *, _Py_Identifier *);
extern int _PyObject_SetAttrId(PyObject *, _Py_Identifier *, PyObject *);
```


### Linked PRs
* gh-120687