Revert "lit: warn when passed invalid pathname" (r210597)

It was pointed out that this breaks the "virtual test discovery"
mechanism, which allows for narming tests in the test exec root.

Reverting until I can figure out how to fix this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211048 91177308-0d34-0410-b5e6-96231b3b80d8