[chore] Get rid of WorkspaceType "probe"

[geropl]

This includes:

• shipping a migration to delete it from all installations
  ´´´
  SELECT COUNT(1)
  FROM d_b_workspace AS ws
  WHERE ws.type = 'probe';
  ´´´
  still reveals a couple of instances
• find last usages and types in code and remove those as well

[easyCZ]

How do we approach nuking the records from the DB? Do we need to make the cleanup work in self-hosted instances also? Or were probes only a cloud concept?

One approach is to write this as a migration which deletes the records, but that in itself may not work too great in terms of the interaction with db-sync.
Alternatively, we run this as a scheduled job on server.

[easyCZ]

Final piece - removal of instances from the DB is here #13477

[easyCZ]

Using an incremental deletion in the garbage collector approach - #13713