decoder_api: removed decoder_clear()

Call ob_clear() in decoder_command_finished() instead of implementing
that call in every decoder plugin.

src/decoder/audiofile_plugin.c

@@ -92,7 +92,6 @@ static int audiofile_decode(struct decoder * decoder, char *path)
 
 	do {
 		if (decoder_get_command(decoder) == DECODE_COMMAND_SEEK) {
-			decoder_clear(decoder);
 			current = decoder_seek_where(decoder) *
 				audio_format.sample_rate;
 			afSeekFrame(af_fp, AF_DEFAULT_TRACK, current);