remove useless code, found by oded

Originally committed as revision 10066 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/nutenc.c

@@ -498,7 +498,6 @@ static int write_packet(AVFormatContext *s, AVPacket *pkt){
 
         if(flags & FLAG_CODED){
             length++;
-            flags &= ~needed_flags;
             flags |=  needed_flags;
         }