build-libcxx: Don't manually install libunwind.dll

libunwind installs the built DLL since
39ad160468e2b6acaa0b2b1ab353079eca4a3b03 (March 2021).

build-libcxx.sh

@@ -108,10 +108,6 @@ build_all() {
             ..
         $BUILDCMD ${CORES+-j$CORES}
         $BUILDCMD install
-        if [ "$type" = "shared" ]; then
-            mkdir -p "$PREFIX/$arch-w64-mingw32/bin"
-            cp lib/libunwind.dll "$PREFIX/$arch-w64-mingw32/bin"
-        fi
         cd ..
     done
     cd ..