avfilter: remove usage of empty header

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavfilter/internal.h

@@ -26,7 +26,6 @@
 
 #include "libavutil/internal.h"
 #include "avfilter.h"
-#include "avfiltergraph.h"
 #include "formats.h"
 #include "framepool.h"
 #include "framequeue.h"