Delete non-existent cloud provider nodes with Ready condition Unknown

[andrewsykim]

What type of PR is this?
/kind bug

What this PR does / why we need it:
Also deletes nodes with Ready condition Unknown if the node does not exist in the cloud provider. This was introduced in #70344 because we assumed that deleted nodes report Ready condition as False instead of Unknown. This PR changes the logic to delete any node that has a Ready condition that is not True and does not exist in the cloud provider (consistent with what we already had prior to merging #70344).

Which issue(s) this PR fixes:
Fixes # #72499

Special notes for your reviewer:
@mtaufen I'm not sure if we should be expecting this condition to be Unknown, but I've opened this PR to bring things back to what they were prior to #70344. Let me know if you prefer another solution.

Does this PR introduce a user-facing change?:

Nodes deleted in the cloud provider with Ready condition `Unknown` should also be deleted on the API server. 

/assign @mtaufen
/sig cloud-provider node

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[mtaufen]

This makes sense to me.
/lgtm