what to do with zpool and physical_disk database tables after expungement

[leftwo]

There is a comment in issue #4787 around what happens to database tables in
a sled that has been removed. This section specifically:

## Database state
...

Besides those direct consumers, physical disks are referenced by the `zpool` 
table.  `zpool` is referenced by `dataset.pool_id` and some inventory tables.  
Again I think we can ignore inventory tables here.  `dataset` is referenced by 
`region.dataset_id` and `region_snapshot.dataset_id`.  I'm assuming that 
`physical_disk`, `zpool`, `dataset`, and `region` are well understood and covered 
by the open tickets around #4719 and Crucible region replacement.

Originally posted by @davepacheco in #4787 (comment)

The current plan for Crucible region replacement does not address what to do with
the zpool and physical_disk tables.

It may not be a problem to just leave them as they are, but I did want to call out that
at the moment, no special action is taken for these tables during the Crucible region
replacement work.

[davepacheco]

We discussed this briefly in today's update call and concluded that:

• eventually, we may want to clean up zpool and physical_disk records, as we want to clean up tons of other kinds of records (e.g., old blueprints, completed sagas, deleted silos/projects/instances/VPCs/etc.); and
• the presence of these records in the meantime is not a problem. e.g., we won't accidentally provision to the zpool because we're already filtering for disks or sleds that are in-service.