Remove duplicate initialiser for cavsvideo_muxer.extensions

The extensions field was initialised first to "cavs", then to "avs".
The name "cavs" is kept since this is used for the format elsewhere
and "avs" is already used for avisynth files.

Originally committed as revision 24538 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/raw.c

@@ -993,7 +993,6 @@ AVOutputFormat cavsvideo_muxer = {
     NULL,
     raw_write_packet,
     .flags= AVFMT_NOTIMESTAMPS,
-    .extensions = "avs",
 };
 #endif