[BE] Remove unused conda-env-Linux-X64 (pytorch#154303)

According to https://github.com/search?type=code&q=conda-env-++repo%3Apytorch%2Fpytorch it's not referenced anywhere and has been replaced with `conda-env-ci` a while ago
Pull Request resolved: pytorch#154303
Approved by: https://github.com/cyyever, https://github.com/Skylion007

Author: malfet