* mail_message.message_read() method is not used anymore since the chatter
refactoring in v9. message_format() is used instead. Tests for this method
are also removed, as they have no use anymore.
* mail_thread.message_set_read() is not used anymore. set_message_done is the
only method used by the new chatter as there is no read state anymore.
* removed an unused parameter in set_message_done, by the way