[ONNX] Clean up outdated skip ort < 1.15 decorator in tests (#105951)
`skip_min_ort_version` is not needed anymore, as the ort version is now officially > 1.15. But the function is kept for future usage.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105951
Approved by: https://github.com/BowenBao, https://github.com/thiagocrepaldi
