Clean up redundant ifdef in list getitem (python#128257)

It's already inside a `Py_GIL_DISABLED` block so the `#else` clause is always unused.

Author: da-woods

Objects/listobject.c

@@ -335,11 +335,7 @@ list_item_impl(PyListObject *self, Py_ssize_t idx)
     if (!valid_index(idx, size)) {
         goto exit;
     }
-#ifdef Py_GIL_DISABLED
     item = _Py_NewRefWithLock(self->ob_item[idx]);
-#else
-    item = Py_NewRef(self->ob_item[idx]);
-#endif
 exit:
     Py_END_CRITICAL_SECTION();
     return item;