Strip GCC5 stuff from PyTorch (#85914)
[This file](https://github.com/pytorch/pytorch/pull/63208/files) indicates that we don't support anything less than GCC 7.5. Given that, let's remove this GCC 5 stuff.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85914
Approved by: https://github.com/ezyang
