Fix the model implementation of `listRetiredPools`.
The `prop_poolRetirement` property expects that `listRetiredPools` will include all pools with non-cancelled retirement certificates, even when those pools never had registration certificates to begin with. Strictly speaking, this expectation is more than we need, as in reality we'll probably never see a case where a pool publishes a retirement certificate without there being a prior registration certificate. (This should be explicitly forbidden by the ledger layer.) However, it turns out to be more convenient to express the property this way, as: 1. It makes the property much simpler to express. We don't need to worry about registration certificates in the setup phase. 2. It makes the SQLite query for implementing `listRetiredPools` much easier to write. Therefore, in this case, we simply update the model to match the behaviour of our SQLite implementation.