bpo-38321: Fix _asynciomodule.c compiler warning (GH-16493)

bpo-38248, bpo-38321: Fix warning:

    modules\_asynciomodule.c(2667):
    warning C4102: 'set_exception': unreferenced label

The related goto has been removed by
commit edad4d89e3.

Modules/_asynciomodule.c

@@ -2657,7 +2657,6 @@ task_step_impl(TaskObj *task, PyObject *exc)
         /* Some other exception; pop it and call Task.set_exception() */
         PyErr_Fetch(&et, &ev, &tb);
 
-set_exception:
         assert(et);
         if (!ev || !PyObject_TypeCheck(ev, (PyTypeObject *) et)) {
             PyErr_NormalizeException(&et, &ev, &tb);