decoder: automatically flush the output buffer after decoder exits

A decoder_flush() invocation was missing in the FLAC plugin, resulting
in casual assertion failures due to a wrong assumption about the last
chunk's audio format.  It's much easier to remove that decoder_flush()
function and make the decoder thread call ob_flush().

src/decoder/aac_plugin.c

@@ -419,8 +419,6 @@ static int aac_stream_decode(struct decoder * mpd_decoder,
 			break;
 	}
 
-	decoder_flush(mpd_decoder);
-
 	faacDecClose(decoder);
 	if (b.buffer)
 		free(b.buffer);
@@ -556,8 +554,6 @@ static int aac_decode(struct decoder * mpd_decoder, char *path)
 			break;
 	}
 
-	decoder_flush(mpd_decoder);
-
 	faacDecClose(decoder);
 	if (b.buffer)
 		free(b.buffer);