x/crypto: disable wantReply in auth-agent-req to comply with OpenSSH

The existing implementation does not comply with OpenSSH, where
the agent forwarding request is sent without wantReply flag.
https://github.com/openssh/openssh-portable/blob/master/ssh.c#L1818
(wantconfirm=0)

ssh/agent/forward.go

@@ -17,14 +17,8 @@ import (
 // ForwardToAgent or ForwardToRemote should be called to route
 // the authentication requests.
 func RequestAgentForwarding(session *ssh.Session) error {
-	ok, err := session.SendRequest("[email protected]", true, nil)
-	if err != nil {
-		return err
-	}
-	if !ok {
-		return errors.New("forwarding request denied")
-	}
-	return nil
+	_, err := session.SendRequest("[email protected]", false, nil)
+	return err
 }
 
 // ForwardToAgent routes authentication requests to the given keyring.