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_api.h

@@ -159,6 +159,4 @@ decoder_data(struct decoder *decoder,
 	     void *data, size_t datalen, float data_time, uint16_t bitRate,
 	     ReplayGainInfo * replayGainInfo);
 
-void decoder_flush(struct decoder *decoder);
-
 #endif