Remove forking completely from `IStore`

Since moving to PBFT, there has been no real need to allow forking of chains. The following methods should be removed: - `IStore.ForkTxNonces()` - `IStore.ForkBlockIndexes()` - `IStore.DeleteChainId()` - `IStore.PruneOutdatedChains()` - `IStore.ListChainIds()` These methods may be provided under `RocksDBStore` temporarily for operational reasons, but should no longer be part of the `IStore` interface. It may be desirable to remove all in one go. Due to the 9c-network chain database being corrupted internally already, it would be nearly impossible to implement additional migration methods while accommodating existing chains. Additionally, we might want to remove `RocksDBStore.MigrateChainDBFromColumnFamilies()` as well. I propose the following migration path: - Remove all interface methods suggested above. - Implement `RocksDBStore.PruneOutdatedChains()` as in #3999. - Automatically prune `RocksDBStore` during the instantiation phase. - This can be checked by calling `ListChainIds()` and see if there is more than 1. - Pruning can be skipped if the `RocksDBStore` is already pruned. - Fix `RocksDBStore.IterateIndexes()`. - As it stands, `RocksDBStore.IterateIndexes()` is bugged, which I believe is the reason why the current 9c-network chains are corrupted. - Once we can assume that a loaded `RocksDBStore` is always pruned, reimplementing `RocksDBStore.IterateIndexes()` becomes trivial. - A reimplementation of `RocksDBStore.IterateIndexes()` may require some optimization as pruning will result in significant performance degradation due to an `Iterator` normally having to iterate from the very beginning even if one is only looking for the last `n` blocks. `Iterator.SeekForPrev()` may be used for this purpose. Once this is done, I presume significant portions of `RocksDBStore` can be refactored easily after a full release cycle (to make sure that all local chains are pruned) making additional migrations feasible for further optimizations.