[PyTorch] Remove build definitions from `build_variables.bzl` (#36602)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36602
`build_variables.bzl` should contain only filelists to make it interpretable between BUCK, Cmake and Bazel build systems.
Test Plan: CI
Reviewed By: dzhulgakov
Differential Revision: D21022886
fbshipit-source-id: 9dd1e289ac502bc325e1223197b6156a316498ba
