Remove post-altair `initialize_beacon_state_from_eth1` from specs

[hwwhww]

(copied from our previous discussion)

There are the post-altair testing initialize_beacon_state_from_eth1 helpers that set previous_version == current_version. It was designed for the testnets.

That said, it seems the previous testnets prefer using phase0 genesis and calling a series of upgrade_to_<fork> at slot 0…

If no one uses it, we may consider removing the post-altair initialize_beacon_state_from_eth1 from the specs.

[ralexstokes]

we do indirectly use these functions in testing, its not a big deal to move them to just testing code, and would simplify the specs so I support doing it

[dapplion]

The genesis generators used in our testing just re-use the upgrade_to_* functions from a phase0 genesis state. The result should be identical. Having to maintain the extra initialize_beacon_state_from_eth1 is just boilerplate.

    if spec.altair_fork_epoch == Some(GENESIS_EPOCH) {
        upgrade_to_altair(&mut state, spec).unwrap();
    }
    if spec.bellatrix_fork_epoch == Some(GENESIS_EPOCH) {
        upgrade_to_bellatrix(&mut state, spec).unwrap();
    }
    if spec.capella_fork_epoch == Some(GENESIS_EPOCH) {
        upgrade_to_capella(&mut state, spec).unwrap();
    }