Remove all devtoolset7 jobs (#21153)
Summary:
These do not work. We'll save time and cpu until someone has the time to fix these.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21153
Differential Revision: D15558601
Pulled By: pjh5
fbshipit-source-id: f9bfe580aa7962a88506f9af0032647f553637a4
