[PyTorch] Delete c10::guts::if_constexpr (#101991)
Now that we have C++17, we should not need this any more.
Differential Revision: [D46078335](https://our.internmc.facebook.com/intern/diff/D46078335/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101991
Approved by: https://github.com/r-barnes, https://github.com/Skylion007
