cluster: cleanup ansible import related things

[dveeden]

What problem does this PR solve?

Follow-up for #2565

This reduces a lot of complexity and code that isn't used or tested often.

What is changed and how it works?

Remove more Ansible import related things.

• This is keeping the Imported field, but sets the key to -.

Questions
Should we check if the imported field is set in the config and throw an error if that's the case?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release notes:

Clusters imported from an ansible installation are not supported anymore

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[dveeden]

/cc @xhebox

[AstroProfundis]

TiDB-Ansible used a different dir scheme on nodes, and when importing to TiUP Cluster it's not capable to change that, so there are many compatibility wraps.

For those old legacy clusters, if these wrapping codes got removed, the cluster might be unable to start after reloading config with new tiup-cluster, (or maybe better, they fail specification validation so nothing actually change on their node, no data loss risk)

I don't know if there are still any of those old clusters left but it's better to confirm. AFAIK some on-premise deployments were of this kind, I don't know if they are still in use.

[dveeden]

@AstroProfundis thanks for the info. This is why I checked "Breaking backward compatibility" and set the release note to mention this.