TH: Defer to ATen's AVX detection code (#34088)
Summary:
As per https://github.com/pytorch/pytorch/issues/22338#issuecomment-593028168, this removes the AVX detection code from TH. Now the environment variable `ATEN_CPU_CAPABILITY` is the only setting needed to disable AVX/AVX2.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34088
Differential Revision: D20236039
Pulled By: ezyang
fbshipit-source-id: eecec64b41a7a6ca7e42c1c2762032eb47af535c
