_pkg.dev: drop the last wire/util code

SumSHA256() and ReaderToBuffer() are not used, CalculateHash() shouldn't be
used and BufferLength() is just to easy with only one user.

_pkg.dev/wire/message.go

@@ -47,7 +47,7 @@ func WriteMessage(w io.Writer, magic protocol.Magic, message Messager) error {
 		return err
 	}
 
-	payloadLen := util.BufferLength(buf)
+	payloadLen := uint32(buf.Len())
 	checksum := checksum.FromBytes(buf.Bytes())
 
 	bw.Write(payloadLen)