build-libcxx: Stop setting LIBCXXABI_TARGET_TRIPLE

It doesn't seem to be strictly necessary right now, in the current
configuration.

build-libcxx.sh

@@ -176,7 +176,6 @@ build_all() {
             -DLIBCXXABI_USE_COMPILER_RT=ON \
             -DLIBCXXABI_ENABLE_EXCEPTIONS=ON \
             -DLIBCXXABI_ENABLE_THREADS=ON \
-            -DLIBCXXABI_TARGET_TRIPLE=$arch-w64-mingw32 \
             -DLIBCXXABI_ENABLE_SHARED=OFF \
             -DLIBCXXABI_LIBCXX_INCLUDES=../../libcxx/build-$arch-$type/include/c++/v1 \
             -DLIBCXXABI_LIBDIR_SUFFIX="" \