wire-mix: remove xxx

wire/msgmixrs.go

@@ -193,9 +193,13 @@ func (msg *MsgMixRS) Expires() int64 {
 	return msg.Expiry
 }
 
-// PrevMsgs returns the previous DC messages seen by the peer.
+// PrevMsgs returns nil.  Previous messages are not needed to perform blame
+// assignment, because of the assumption that all previous messages must have
+// been received for a blame stage to be necessary.  Additionally, a
+// commitment to the MixMsgRS hash is included in the KE, and future message
+// hashes are not available at that time.
 func (msg *MsgMixRS) PrevMsgs() []chainhash.Hash {
-	return nil // XXX
+	return nil
 }
 
 // Sid returns the session ID.