fftools/ffmpeg: stop inventing fake source information

This code was supposed to affect copying stream dispositions, but it
does not achieve that after bd55552d69, since dispositions are set
earlier.

fftools/ffmpeg.c

@@ -3170,21 +3170,6 @@ static int transcode_init(void)
     InputStream *ist;
     char error[1024] = {0};
 
-    for (i = 0; i < nb_filtergraphs; i++) {
-        FilterGraph *fg = filtergraphs[i];
-        for (j = 0; j < fg->nb_outputs; j++) {
-            OutputFilter *ofilter = fg->outputs[j];
-            if (!ofilter->ost || ofilter->ost->source_index >= 0)
-                continue;
-            if (fg->nb_inputs != 1)
-                continue;
-            for (k = nb_input_streams-1; k >= 0 ; k--)
-                if (fg->inputs[0]->ist == input_streams[k])
-                    break;
-            ofilter->ost->source_index = k;
-        }
-    }
-
     /* init framerate emulation */
     for (i = 0; i < nb_input_files; i++) {
         InputFile *ifile = input_files[i];