fftools/ffmpeg_filter: drop a redundant error message

In case no decoder is available, dec_open() called from ist_use() will
fail with 'Decoding requested, but no decoder found', so this check is
redundant.

fftools/ffmpeg_filter.c

@@ -1201,12 +1201,6 @@ static int configure_input_audio_filter(FilterGraph *fg, InputFilter *ifilter,
 static int configure_input_filter(FilterGraph *fg, InputFilter *ifilter,
                                   AVFilterInOut *in)
 {
-    if (!ifilter->ist->dec) {
-        av_log(NULL, AV_LOG_ERROR,
-               "No decoder for stream #%d:%d, filtering impossible\n",
-               ifilter->ist->file_index, ifilter->ist->st->index);
-        return AVERROR_DECODER_NOT_FOUND;
-    }
     switch (avfilter_pad_get_type(in->filter_ctx->input_pads, in->pad_idx)) {
     case AVMEDIA_TYPE_VIDEO: return configure_input_video_filter(fg, ifilter, in);
     case AVMEDIA_TYPE_AUDIO: return configure_input_audio_filter(fg, ifilter, in);