CODEC_ID_XVID is not used anywhere, remove it at next major bump

Originally committed as revision 22511 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -125,7 +125,9 @@ enum CodecID {
     CODEC_ID_QDRAW,
     CODEC_ID_VIXL,
     CODEC_ID_QPEG,
+#if LIBAVCODEC_VERSION_MAJOR < 53
     CODEC_ID_XVID,
+#endif
     CODEC_ID_PNG,
     CODEC_ID_PPM,
     CODEC_ID_PBM,