amr demuxer: do not set AVCodecContext.frame_size.

it is not necessary.

libavformat/amr.c

@@ -100,14 +100,12 @@ static int amr_read_header(AVFormatContext *s)
         st->codec->codec_tag = MKTAG('s', 'a', 'w', 'b');
         st->codec->codec_id = CODEC_ID_AMR_WB;
         st->codec->sample_rate = 16000;
-        st->codec->frame_size = 320;
     }
     else
     {
         st->codec->codec_tag = MKTAG('s', 'a', 'm', 'r');
         st->codec->codec_id = CODEC_ID_AMR_NB;
         st->codec->sample_rate = 8000;
-        st->codec->frame_size = 160;
     }
     st->codec->channels = 1;
     st->codec->codec_type = AVMEDIA_TYPE_AUDIO;