Remove the following unstable functions:

* ``PyUnstable_Replace_Executor()``
* ``PyUnstable_SetOptimizer()``
* ``PyUnstable_GetOptimizer()``
* ``PyUnstable_GetExecutor()``
* ``PyUnstable_Optimizer_NewCounter()``
* ``PyUnstable_Optimizer_NewUOpOptimizer()``

Patch by Victor Stinner.
