HACK: don't override DX when running make

The test runner shouldn't be overriding the location of DX when
running make, if it wants it set in the environment it should do
it after runing make.  For now just unset it when running make.

Test: art/test/testrunner/run_build_test_target.py -j110 art-gtest
Change-Id: Ic97b0f1edc854da4e352290ff192a0389766403a
2 files changed