Revert "Fix llvm-lit script generation in libcxx."

This reverts commit 4ad71811d45268d81b60f27e3b8b2bcbc23bd7b9.

There is a bot that is checking out libcxx and lit with nothing
else and then running lit.py against the test tree.  Since there's
no LLVM source tree, there's no LLVM CMake.  CMake actually
reports this as a warning saying unsupported libcxx configuration,
but I guess someone is depending on it anyway.

llvm-svn: 313607