avfilter/avfilter: Remove unused feature to add pads in the middle

Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/vf_stack.c

@@ -123,7 +123,7 @@ static av_cold int init(AVFilterContext *ctx)
         if (!pad.name)
             return AVERROR(ENOMEM);
 
-        if ((ret = ff_insert_inpad(ctx, i, &pad)) < 0) {
+        if ((ret = ff_append_inpad(ctx, &pad)) < 0) {
             av_freep(&pad.name);
             return ret;
         }