[relay] Improve relay messages (#2574)

Co-authored-by: Zoltán Papp <zoltan.pmail@gmail.com>

relay/messages/auth/auth.go

@@ -1,3 +1,4 @@
+// Deprecated: This package is deprecated and will be removed in a future release.
 package auth
 
 import (
@@ -30,15 +31,6 @@ type Msg struct {
 	AdditionalData []byte
 }
 
-func (msg *Msg) Marshal() ([]byte, error) {
-	var buf bytes.Buffer
-	enc := gob.NewEncoder(&buf)
-	if err := enc.Encode(msg); err != nil {
-		return nil, fmt.Errorf("encode Msg: %w", err)
-	}
-	return buf.Bytes(), nil
-}
-
 func UnmarshalMsg(data []byte) (*Msg, error) {
 	var msg *Msg