do not write id3v2 header when writing .mp2 files

Originally committed as revision 9924 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/mp3.c

@@ -608,7 +608,7 @@ AVOutputFormat mp2_muxer = {
     0,
     CODEC_ID_MP2,
     0,
-    mp3_write_header,
+    NULL,
     mp3_write_packet,
     mp3_write_trailer,
 };