fftools/ffmpeg: stop using packet pts for decoded audio frame pts

If input packets have timestamps, they will be propagated to output
frames by the decoder, so at best this block does not do anything.

There can also be an arbitrary amount of delay between packets sent to
the decoder and decoded frames (e.g. due to decoder's intrinsic delay or
frame threading), so deriving any timestamps from packet properties is
wrong.

fftools/ffmpeg.c

@@ -911,9 +911,6 @@ static int decode_audio(InputStream *ist, AVPacket *pkt, int *got_output,
 
     if (decoded_frame->pts != AV_NOPTS_VALUE) {
         decoded_frame_tb   = ist->st->time_base;
-    } else if (pkt && pkt->pts != AV_NOPTS_VALUE) {
-        decoded_frame->pts = pkt->pts;
-        decoded_frame_tb   = pkt->time_base;
     }else {
         decoded_frame->pts = ist->dts;
         decoded_frame_tb   = AV_TIME_BASE_Q;