Cleanup clear_static_tp_subclasses() (#106276)

Only iterate on the dictionary if Python is built with assertions:
subclass is only needed when Python is built with assertions.

Author: vstinner

Objects/typeobject.c

@@ -5009,6 +5009,7 @@ clear_static_tp_subclasses(PyTypeObject *type)
        going to leak.  This mostly only affects embedding scenarios.
      */
 
+#ifndef NDEBUG
     // For now we just do a sanity check and then clear tp_subclasses.
     Py_ssize_t i = 0;
     PyObject *key, *ref;  // borrowed ref
@@ -5021,6 +5022,7 @@ clear_static_tp_subclasses(PyTypeObject *type)
         assert(!(subclass->tp_flags & _Py_TPFLAGS_STATIC_BUILTIN));
         Py_DECREF(subclass);
     }
+#endif
 
     clear_tp_subclasses(type);
 }