refactor(nns): Remove legacy options using protobuf for NNS Governance init

[jasonz-dfinity]

Remove 2 legacy options in ic_nns_init:

• governance_pb_file: using a pb file to instantiate NNS Governance. There doesn't seem to be anyone using it. There are also better options: initial_neurons allow arbitrary neurons and the default initializes 3 neurons.
• output_initial_state_candid_only: this supposedly creates "candid files" (although 2 out of 4 are protobuf). However, the functionalities to use those files seem incomplete.

Overall, the removals don't affect production code. We can easily revert if it turns out they are used.

[github-actions]

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[github-actions]

If this pull request affects the behavior of any canister owned by
the Governance team, remember to update the corresponding
unreleased_changes.md file(s).

To acknowldge this reminder (and unblock the PR), dismiss this
code review by going to the bottom of the pull request page, and
supply one of the following reasons:

1. Done.

2. No canister behavior changes.

No canister behavior changes

[max-dfinity]

Will there be a later PR to add a candid file? Or is this utility without much use? Would it be used in recovery?

[jasonz-dfinity]

Will there be a later PR to add a candid file? Or is this utility without much use? Would it be used in recovery?

I'm not planning to do so. I doubt there is a good way to do "recovery" based on the output files, since there doesn't seem to be a way to store them. There also doesn't seem to be a way to use the CMC file. My guess is that it was added for testing in static testnets for faster iterations.