audio_format: remove the packed S24 format

For simplicity, the MPD core should not have to deal with packing.  It
is rarely used, and those plugins that need it should use the
pcm_export library instead.

src/pcm_volume.c

@@ -158,7 +158,6 @@ pcm_volume(void *buffer, size_t length,
 	const void *end = pcm_end_pointer(buffer, length);
 	switch (format) {
 	case SAMPLE_FORMAT_UNDEFINED:
-	case SAMPLE_FORMAT_S24:
 	case SAMPLE_FORMAT_DSD:
 	case SAMPLE_FORMAT_DSD_OVER_USB:
 		/* not implemented */