feat(registry): Obsolete legacy ECDSA configs

[aterga]

This PR is a follow-up to the chain-key config generalization worked that took place last year. Now that chain key has worked on the mainnet for a few months, the clients are ready, we simplify the API.

Notes on changed tests:

• Most tests were generalized from legacy ECDSA to chain key in scope of the original work, so the remaining legacy ones are deleted in this PR, not ported (those were marked TODO[NNS1-3102]: Remove this test.).
• Since we no longer need legacy ECDSA <--> chain key conversions, the corresponding tests were removed.
• Tests that covered legacy ECDSA and chain key configs being specified together were removed, as not legacy ECDSA is always forbidden.
• In one instance, I de-duplicate tests to avoid obvious redundancy (test_disallow_chain_key_signing_disable_and_enable_for_same_key and enable_and_disable_signing_lists_should_not_have_same_keys_in_single_request)

[github-actions]

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Done.