lavf: move "MP3 " fourcc from riff to nut

Original commit, 7b24f9b4, says it was added because it is used in libnut.

libavformat/nut.c

@@ -145,6 +145,7 @@ const AVCodecTag ff_nut_audio_tags[] = {
     { AV_CODEC_ID_PCM_U32LE,        MKTAG('P', 'U', 'D', 32 ) },
     { AV_CODEC_ID_PCM_U8,           MKTAG('P', 'U', 'D',  8 ) },
     { AV_CODEC_ID_PCM_S16LE_PLANAR, MKTAG('P', 'S', 'P', 16 ) },
+    { AV_CODEC_ID_MP3,              MKTAG('M', 'P', '3', ' ') },
     { AV_CODEC_ID_NONE,             0                         }
 };
 

libavformat/riff.c

@@ -348,7 +348,6 @@ const AVCodecTag ff_codec_wav_tags[] = {
     // for NuppelVideo (nuv.c)
     { AV_CODEC_ID_PCM_S16LE, MKTAG('R', 'A', 'W', 'A') },
     { AV_CODEC_ID_MP3,       MKTAG('L', 'A', 'M', 'E') },
-    { AV_CODEC_ID_MP3,       MKTAG('M', 'P', '3', ' ') },
     { AV_CODEC_ID_NONE,      0 },
 };