remove DEMANGLE_SUPPORT from build-ffmpeg.sh.

wasm/build-scripts/build-ffmpeg.sh

@@ -31,7 +31,6 @@ FLAGS=(
   -s EXPORTED_FUNCTIONS="[_main]"  # export main and proxy_main funcs
   -s EXPORTED_RUNTIME_METHODS="[FS, cwrap, ccall, setValue, writeAsciiToMemory]"   # export preamble funcs
   -s INITIAL_MEMORY=2146435072                  # 64 KB * 1024 * 16 * 2047 = 2146435072 bytes ~= 2 GB
-  -s DEMANGLE_SUPPORT=1
   --pre-js wasm/src/pre.js
   --post-js wasm/src/post.js
   $OPTIM_FLAGS