Remove toplevel src/ and test/ folders.

These were added in 947e2d61ece32f936f1df8c93a22d7489d54abdc by
@jroelofs, as far as I can tell that wasn't intentional. (The same files
exist below sources/cxx-stl/llvm-libc++abi too; that's the correct
place.)

Change-Id: I5d195969996247707d300b64e7c230fb97a72b36