remove cylc scp-transfer

[oliver-sanders]

As per https://cylc.github.io/cylc-admin/proposal-config-changes.html#job-platformsscp-command---obsolete

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).
• Does not need tests (why?).
• No change log entry required (why? e.g. invisible to users).
• No documentation update required. - greped the docs, no reference

[hjoliver]

I'll save the assigned reviewers some time. git grep doesn't reveal any other reference to this. I also checked, there's no reference to it in cylc-doc.