avconv: remove some codec-specific hacks

The problem they are supposed to fix is handled in riff.c, so those
hacks are pointless.

avconv.c

@@ -2046,10 +2046,6 @@ static int transcode_init(OutputFile *output_files,
                 codec->frame_size = icodec->frame_size;
                 codec->audio_service_type = icodec->audio_service_type;
                 codec->block_align= icodec->block_align;
-                if(codec->block_align == 1 && codec->codec_id == CODEC_ID_MP3)
-                    codec->block_align= 0;
-                if(codec->codec_id == CODEC_ID_AC3)
-                    codec->block_align= 0;
                 break;
             case AVMEDIA_TYPE_VIDEO:
                 codec->pix_fmt = icodec->pix_fmt;