Delete genesisSlotNo and genesisBlockNo

[edsko]

These two functions should be gone for two reasons:

1. They are very confusing. genesisPoint (admittedly not defined here, but in ouroboros-network) refers to the point corresponding to the genesis config, i.e., before the first block on the chain. By contrast, genesisSlotNo and genesisBlockNo refer to the first block on the chain. This has led to a host of off-by-one errors.

2. Nothing in the core infrastructure should depend on the exact values of the slot number and block number of the first block on the chain. Instead, such details should live in specific ledger implementations (Shelley, Byron, whatever).

[edsko]

This was closed in #79.