Remove code that's almost always dead, and harmful if not.

lit's util.which() would check if the passed-in path existed directly,
and if so return it as-is.  This is never the case when running llvm's, clang's,
or lld's tests normally.  But when running `./llvm-lit path/to/clang/test`
with a cwd of llvm-build/bin, this if would detect that clang exists at path
'clang' and return 'clang' as the discovered clang binary -- and then lit would
use the " clang " -> "*** Do not use 'clang' in tests, use '%clang'. ***"
substitution to replace that with a broken test.  By removing this early
return, lit ends up with the usual absolute path and everything works even
in this uncommon case.

llvm-svn: 330672