Deprecate Message::$format (mostly)

Deprecate Message::$format (mostly)

Message::__toString() used the same formatting mode that the last
explicit transformation used:

    $msg = new Message( 'foo' );
    echo $msg; // escaped
    echo $msg->plain();
    echo $msg; // not escaped

This is not particularly useful and makes code review hard, so let's
get rid of it.

The same behavior with $msg->toString() is left intact (and logged)
for now.

Bug: T146416
Change-Id: Ia9b2a1dcf09d52348b2c6d8299fd849b809f6e74