This reverts commit 63f0fdc2628dfb2f52ed5a92e579f99261d946ed.
Since f1431bbfb17cd7167adda9fc8521bb6eec52c300, this environment
variable is always set up by lit itself, so individual test suites
don't need to set it.
Differential Revision: https://reviews.llvm.org/D149356