decoder_internal: don't call input_stream_buffer()

This is not necessary since all relevant input plugins have been moved
to the I/O thread, and there is no remaining useful buffer()
implementation.  This also fixes a busy loop when playing radio.

src/decoder_internal.h

@@ -70,7 +70,7 @@ struct decoder {
  * @return the chunk, or NULL if we have received a decoder command
  */
 struct music_chunk *
-decoder_get_chunk(struct decoder *decoder, struct input_stream *is);
+decoder_get_chunk(struct decoder *decoder);
 
 /**
  * Flushes the current chunk.