avfilter/f_metadata: remove unused headers

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

libavfilter/f_metadata.c

@@ -28,10 +28,8 @@
 #include "libavutil/avassert.h"
 #include "libavutil/avstring.h"
 #include "libavutil/eval.h"
-#include "libavutil/fifo.h"
 #include "libavutil/internal.h"
 #include "libavutil/opt.h"
-#include "libavutil/pixelutils.h"
 #include "avfilter.h"
 #include "audio.h"
 #include "formats.h"