Don't prompt to clean no workflows.

[hjoliver]

This is a small change with no associated Issue.

Master:

$ cylc clean 'sagittarius-a*'
Would clean the following workflows:
Remove these workflows (y/n): 

This branch:

$ cylc clean 'sagittarius-a*'
No workflows matching sagittarius-a*

Requirements 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.
• Does not need tests (why?).
• Appropriate change log entry included.
• No documentation update required.

[hjoliver]

IMO we don't really need a test to check that an unnecessary prompt doesn't get issued.

[hjoliver]

One review will do for this.