avformat/webmdashenc: Remove unnecessary header

avio_internal.h has been included in this muxer since the beginning and
was never needed.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/webmdashenc.c

@@ -31,7 +31,6 @@
 #include <string.h>
 
 #include "avformat.h"
-#include "avio_internal.h"
 #include "matroska.h"
 
 #include "libavutil/avstring.h"