[3.13] Clean up redundant ifdef in list getitem (pythonGH-128257) (python#128276)

Clean up redundant ifdef in list getitem (pythonGH-128257)

It's already inside a `Py_GIL_DISABLED` block so the `#else` clause is always unused.
(cherry picked from commit 42f7a00)

Co-authored-by: da-woods <[email protected]>

Author: miss-islington

Objects/listobject.c

@@ -373,11 +373,7 @@ 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;