Make QDBusConnectionPrivate::send return bool

It used to return the sent message's serial ID, but we never used that.
So simply use boolean instead.

Change-Id: Ic5d393bfd36e48a193fcffff13b73753ccf47759
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>
Reviewed-by: Albert Astals Cid <aacid@kde.org>