decoder/dsd: use decoder_read_full() where appropriate

Addresses Mantis ticket 0004015.

[mk: use decoder_read_full() only when needed, and a few formal
changes]

src/decoder/DsdLib.hxx

@@ -58,10 +58,6 @@ public:
 	}
 };
 
-bool
-dsdlib_read(Decoder *decoder, InputStream &is,
-	    void *data, size_t length);
-
 bool
 dsdlib_skip_to(Decoder *decoder, InputStream &is,
 	       int64_t offset);