task pool: demote "already finished" log message

[oliver-sanders]

Remove Demote an unnecessary log message:

• This message tells us that the task's outcome was loaded from the DB rather than from memory.
• This information isn't pertinent information for the user.
• This message duplicates information that is already in the log.
• This message gets duplicated once for each trigger, potentially resulting in a very large number of messages.

Accept or close.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[hjoliver]

My only question here is would this better demoted to DEBUG, rather than deleted?

Automatically bringing historical tasks back to n=0 is a pretty important and subtle thing, so it should be easily traceable if things go wrong.

[oliver-sanders]

Oh all right.