common: Only use an existing session bus

As soon as there is a dbus user session a session bus should be
available. Only verify that this exists (failling if it doesn't) rather
then trying to workaround things by autolaunching a new bus.

Reviewed-by: Luis Araujo <luis.araujo@collabora.co.uk>
Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D3663