Remove the following unstable functions:
* ``PyUnstable_Replace_Executor()``
* ``PyUnstable_SetOptimizer()``
* ``PyUnstable_GetOptimizer()``
* ``PyUnstable_GetExecutor()``
* ``PyUnstable_Optimizer_NewCounter()``
* ``PyUnstable_Optimizer_NewUOpOptimizer()``
Patch by Victor Stinner.