android/ffmpeg: remove redundant -j4

-j4 should be configure in do-detect-env.sh

android/contrib/tools/do-compile-ffmpeg.sh

@@ -310,7 +310,7 @@ echo "--------------------"
 echo "[*] compile ffmpeg"
 echo "--------------------"
 cp config.* $FF_PREFIX
-make -j4 $FF_MAKE_FLAGS > /dev/null
+make $FF_MAKE_FLAGS > /dev/null
 make install
 mkdir -p $FF_PREFIX/include/libffmpeg
 cp -f config.h $FF_PREFIX/include/libffmpeg/config.h