Reconfigurator: Delete `Dataset` records for expunged zones

[jgallagher]

#6065 ensures we create Dataset records for Omicron zones that have durable datasets, but nothing today removes records for expunged zones. @smklein noted this is more complicated for crucible zones:

e.g., the region replacement task currently does a big join of "given a region, look up the dataset, and given that dataset, look up the zpool, and given the zpool, look up the physical disk and check the policy"
so, leaving that query as-is, [we] can't actually delete the dataset rows until the region replacement task has finished

This is strongly related to #6042 (separating zone / dataset management) and lightly related to #6051 (we also never clean up Zpool rows).