remove init-genesis-state command and supporting code from horizon

[tamirms]

init-genesis-state is a deprecated command that was only used when we ingested from the stellar core postgres DB. It is no longer useful now that we ingest solely from captive core / galexie

the scenario in which init-genesis-state is useful is if you have a validator in a standalone network and horizon is ingesting from the validator's postgres db. In that case, horizon can use init-genesis-state to ingest the initial state of the stellar standalone network: https://github.com/stellar/go/blob/master/ingest/genesis.go#L10 . Once the initial state is ingested into the horizon db, horizon can ingest ledger 2 and onwards by querying the validator's postgres db.