build-libcxx: Stop setting -Wno-dll-attribute-no-redeclaration when building libunwind

This is set by libunwind itself since
df6d2e8ab1a4212284e4763724a2211df2c7394a in llvm-project.

build-libcxx.sh

@@ -105,8 +105,6 @@ build_all() {
             -DLIBUNWIND_ENABLE_SHARED=$SHARED \
             -DLIBUNWIND_ENABLE_STATIC=$STATIC \
             -DLIBUNWIND_ENABLE_CROSS_UNWINDING=FALSE \
-            -DCMAKE_CXX_FLAGS="-Wno-dll-attribute-on-redeclaration" \
-            -DCMAKE_C_FLAGS="-Wno-dll-attribute-on-redeclaration" \
             ..
         $BUILDCMD ${CORES+-j$CORES}
         $BUILDCMD install