QDBusConnectionPrivate::ObjectTreeNode: remove pointless empty dtor

It just prevents the compiler from synthesizing move
special member functions, something that is very much
desired, seeing as there's a QVector member.

Change-Id: I4daabb380cd73dcacf3f514827b84562767a7a20
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>