Stop adding the toplevel c++ include symlink

As long as we pass --sysroot explicitly in clang-target-wrapper.sh,
this isn't necessary. And since clang SVN r329946. it's no longer
necessary when not specifying the sysroot (and having it is in
fact harmful).

build-libcxx.sh

@@ -150,5 +150,3 @@ for arch in $ARCHS; do
     cd ..
 done
 cd ..
-
-ln -sf ../generic-w64-mingw32/include/c++ $PREFIX/include/c++