slave: Don't stop when finalized

The slave is already stopped by the time it's finalized in all cases,
and we do a ref/unref inside gdm_slave_stop, which is illegal at
finalization time.

The net effect is that we end up printing warnings about ref_count > 0
instead of doing any stopping.
gdm-launch-environment: Remove duplicate DISPLAY set

GdmSession already sets this when setting up the environment.
session-worker-job: Log the session worker to the journal