[ML] Deleting a regression analytics job does not delete its state

[sophiec20]

Found in 7.5.0-SNAPSHOT "build_hash" : "ecb20ebc6cb155be37ecdc68ca8a178531744758", "build_date" : "2019-10-10T09:21:05.340310Z",

When deleting an ML data frame analytics regression job, the state document is not removed form .ml-state. However the progress document is. There are no obvious errors in the log.

This means that re-creating a job with the same name will attempt to load state and continue from where it left off -- even though this is technically a different job.

Marking as non-issue because it has not been released and should not appear in release notes.

[elasticmachine]

Pinging @elastic/ml-core (:ml)