gh-119740: Remove deprecated trunc delegation (#119743)

Remove the delegation of `int` to the `__trunc__` special method: `int` will now only delegate to `__int__` and `__index__` (in that order). `__trunc__` continues to exist, but its sole purpose is to support `math.trunc`.

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Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>