[Proton-J] Removing unused methods in the Message interface

[Rajith Muditha Attapattu]

This is a courtesy notice that I will be removing the following methods
from the Proton Message interface in 48 hrs unless I hear any objections.

I believe Robbie has already mentioned this in the past.

Rajith

    Object save();

    String toAMQPFormat(Object value);

    Object parseAMQPFormat(String value);

    void setMessageFormat(MessageFormat format);

    MessageFormat getMessageFormat();

    void clear();

    MessageError getError();