avfilter/avfilter: Remove unused buffer

Unused since the removal of ff_get_ref_perms_string() in
a05a44e205.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/avfilter.c

@@ -49,7 +49,6 @@ const char av_filter_ffversion[] = "FFmpeg version " FFMPEG_VERSION;
 
 void ff_tlog_ref(void *ctx, AVFrame *ref, int end)
 {
-    av_unused char buf[16];
     ff_tlog(ctx,
             "ref[%p buf:%p data:%p linesize[%d, %d, %d, %d] pts:%"PRId64" pos:%"PRId64,
             ref, ref->buf, ref->data[0],