audio_format: remove SAMPLE_FORMAT_DSD_OVER_USB

DSD-over-USB should not be a MPD core format, because it is not a
"natural" format; it is just a temnporary over-the-wire format.  This
format has been implemented in pcm_export, and does not need to be
supported by pcm_convert.

src/decoder/flac_pcm.c

@@ -103,7 +103,6 @@ flac_convert(void *dest,
 
 	case SAMPLE_FORMAT_FLOAT:
 	case SAMPLE_FORMAT_DSD:
-	case SAMPLE_FORMAT_DSD_OVER_USB:
 	case SAMPLE_FORMAT_UNDEFINED:
 		/* unreachable */
 		assert(false);