bpo-43795: Remove Py_FrozenMain from the Limited API & Stable ABI (GH-26241)

 bpo-43795: Remove Py_FrozenMain from the Limited API & Stable ABI  (GH-26241)

Py_FrozenMain was added to the Limited C API in [bpo-42591]() (3.10.0a4);
but to fix that issue it would be enough to add it to the regular C API.

The function is undocumented, tests were added very recently ([bpo-44131]()),
and most importantly, it is not present in all builds of Python, as
the linker sometimes omits it as unused.
It should be added back when these issues are fixed.

Note that this does not affect Python's regular C API.