Try to remove dbBlocks from module Test.Ouroboros.Storage.LedgerDB.OnDisk

While working on https://github.com/input-output-hk/ouroboros-network/pull/3610 we discovered a problem where it is possible to change the current chain to contain blocks with invalid payload after performing a switch command in the tests of Test.Ouroboros.Storage.LedgerDB.OnDisk. See:

```
dbBlocks: { (hash 0, slot 0) -> valid }
dbState : [ (hash 0, slot 0), LedgerDB valid ]

Switch [ Block (hash 0) (slot 0) invalid ]

dbBlocks: { (hash 0, slot 0) -> invalid }
dbState : [ (hash 0, slot 0), LedgerDB valid ]
```

This task is about analyzing whether dbBlocks can be removed and if so, removing it. If it cannot be removed, then we need to document the reasons why.