remove: enable command for stopping workflows

[oliver-sanders]

• A documented use case for cylc remove is "orphaning" stuck submissions, e.g. jobs that we can no longer poll or kill due to platform or network issues.
• It is conceivable that we might need to do this whilst a workflow is shutting down

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.