ffprobe: stop setting AVCodecContext.framerate

That field is supposed to be exported by decoders, it makes no sense for
a user to set it.

fftools/ffprobe.c

@@ -3034,7 +3034,6 @@ static int open_input_file(InputFile *ifile, const char *filename,
             }
 
             ist->dec_ctx->pkt_timebase = stream->time_base;
-            ist->dec_ctx->framerate = stream->avg_frame_rate;
 
             if (avcodec_open2(ist->dec_ctx, codec, &opts) < 0) {
                 av_log(NULL, AV_LOG_WARNING, "Could not open codec for input stream %d\n",