Cleanup: Zpools rows never die in CRDB #6051
Description
So, fortunately, when we allocate things to physical disks, we check whether or not the corresponding physical disk is expunged or not. This means we won't actually use zpools from expunged disks.
However, we aren't currently marking those zpool rows "deleted" at any point in time. We should. Note that this can't be done immediately at expungement time, since the region_replacement task tries to monitor for non-deleted disks to decide what data to act on.