[BE] Delete `C10_IS_TRIVIALLY_COPYABLE` (#120120)
It's not used anywhere in PyTorch after custom implementation of `c10::optional` is gone, and it's not used by the repo as well, see https://github.com/search?type=code&q=C10_IS_TRIVIALLY_COPYABLE+org%3Apytorch
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120120
Approved by: https://github.com/Skylion007, https://github.com/albanD, https://github.com/huydhn
