Remove final dependency on build-time snark_keys library

[mrmr1993]

This PR tweaks the logic in Genesis_ledger_helper so that it doesn't rely on having access to the Snark_keys library. This should be a no-op for all of our current use cases.

By deleting the check here, this change now allows us to provide arbitrary constraint system constants with proof_level=Full in the runtime configuration. This will generate new p/v keys at startup if they are not found, which is obviously not desirable; until we have fixed this and purged the last remaining uses of Genesis_constants.*.compiled, this check should remain in place.

Checklist:

• Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• All tests pass (CI will check this if you didn't)
• Serialized types are in stable-versioned modules
• Does this close issues? List them:

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