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
diff --git a/Include/cpython/pythonrun.h b/Include/cpython/pythonrun.h
index ffca0e4..2e72d08 100644
--- a/Include/cpython/pythonrun.h
+++ b/Include/cpython/pythonrun.h
@@ -118,3 +118,4 @@ PyAPI_FUNC(PyObject *) PyRun_FileFlags(FILE *fp, const char *p, int s, PyObject
 /* Stuff with no proper home (yet) */
 PyAPI_FUNC(char *) PyOS_Readline(FILE *, FILE *, const char *);
 PyAPI_DATA(PyThreadState*) _PyOS_ReadlineTState;
+PyAPI_DATA(char) *(*PyOS_ReadlineFunctionPointer)(FILE *, FILE *, const char *);