slapos_pdm: Drop needless cleanup on Software Release

The state of Software Release being cleaned up or not is really never
known, so simply drop the concept.

Use case to cover, is some developer having upgrade_scope=auto set and
installing archived SRs - it IS expected that such SR would be cleaned
if not used.
5 jobs for fix/computer-archived-sr-cleanup in 0 seconds
Status Job ID Name Coverage
  External
failed SLAPOS-MASTER-LUKE

09:46:50

failed SLAPOS-MASTER-LUKE

08:45:57

failed SLAPOS-MASTER-LUKE

12:46:07

passed SLAPOS-MASTER-LUKE

07:14:14

failed SLAPOS-MASTER-LUKE

06:10:50