Remove environment variables from driver tests

Summary:
Remove some of dangerous environmental variables from clang/Driver tests.
Driver tests should not rely on preset value of these variables and may
actually fail because of them.
They cannot be removed in test/lit.cfg because we still need to support
relocatable SDKs and other overwrite for other clang tests.

Reviewers: bogner

Subscribers: rnk, cfe-commits

Differential Revision: http://reviews.llvm.org/D7135

llvm-svn: 240574