gh-106084: Remove _PyObject_RealIsInstance() function (#106106)

Remove the following functions from the public C API:

* _PyObject_RealIsInstance()
* _PyObject_RealIsSubclass()
* _Py_add_one_to_index_F()
* _Py_add_one_to_index_C()

Move _PyObject_RealIsInstance() and _PyObject_RealIsSubclass() to the
internal C API (pycore_abstract.h) and no longer export their symbols
(in libpython).

Make _Py_add_one_to_index_F() and _Py_add_one_to_index_C() functions
static: no longer export them.