bpo-43868: Remove PyOS_ReadlineFunctionPointer from the stable ABI list (GH-25442)

The inclusion of PyOS_ReadlineFunctionPointer in python3dll.c was a mistake.

According to PEP 384:
> functions expecting FILE* are not part of the ABI, to avoid depending
> on a specific version of the Microsoft C runtime DLL on Windows.

https://bugs.python.org/issue43868