Never manually connect to the destroyed signal of any object.

The web channel always connects to that signal automatically for
internal book-keeping purposes. Thus we do not have to connect to the
signal for wrapped objects. This simplifies the tests and reduces the
IPC traffic for wrapped objects.

Change-Id: Iaf8d9ce0b87874917cdcdf9013e21a53ee36b53a
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>