Remove run dir housekeeping

[datamel]

Small change, removes run directory rolling archive length from config and associated functionality. This has already been removed in Cylc 7, see #3326.

These changes close #4033

• 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).
• Does not need tests (Removing code).
• No change log entry required (why? This has been removed in Cylc7 and so users will not expect functionality in Cylc8).
• No documentation update required - can not see any reference to this in docs, auto documentation will update.
• No dependency changes.

[hjoliver]

Functional tests possibly still running in the timed-out batch all pass locally on this branch.

[hjoliver]

One review will do for this, thanks @datamel 👍