remove PYTHON_VERSION (#25494)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25494
As far as I can tell, we don't use this anywhere in our CI scripts?
[Diff of processed config](https://gist.github.com/suo/be4ca818afdcb3184f5c61c92f4a4c81)
Test Plan: Imported from OSS
Differential Revision: D17139125
Pulled By: suo
fbshipit-source-id: ff2d025c220a420cda08502eda9fc7d41477e103
