bpo-41117: Cleanup subtract_refs() (GH-27593)

subtract_refs() reuses the 'op' variable rather than calling the
FROM_GC() macro twice.

Issue reported by William Pickard.

Modules/gcmodule.c

@@ -479,9 +479,9 @@ subtract_refs(PyGC_Head *containers)
     for (; gc != containers; gc = GC_NEXT(gc)) {
         PyObject *op = FROM_GC(gc);
         traverse = Py_TYPE(op)->tp_traverse;
-        (void) traverse(FROM_GC(gc),
-                       (visitproc)visit_decref,
-                       op);
+        (void) traverse(op,
+                        (visitproc)visit_decref,
+                        op);
     }
 }