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_convert.h

@@ -42,9 +42,6 @@ struct pcm_convert_state {
 	/** the buffer for converting the sample format */
 	struct pcm_buffer format_buffer;
 
-	/** the buffer for converting to/from packed samples */
-	struct pcm_buffer pack_buffer;
-
 	/** the buffer for converting the channel count */
 	struct pcm_buffer channels_buffer;
 };