Remove useless text editor config.

[hjoliver]

This is a small change with no associated Issue:

• remove "your favourite text editor" config items, and associated code.

The primary reasons for having a configurable editor in Cylc were the cylc trigger --edit and cylc edit commands, neither of which exist any more. (Although the config reference docs still say it is to configure "your favourite editor for editing workflow definitions").

The only remaining use of the configured editor is:

• cylc cat-log --mode=edit
• cylc view (which weirdly defaults to edit instead of stdout)

These are essentially pointless - if you want to view output in your editor, direct the stdout to a file first, or to your editor's stdin.

NB: the config change proposal notes that we agreed in the workshop to keep the editor config but discourage its use. Maybe we had not decided the fate of cylc edit at that point though.

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 (code removal only).
• Appropriate change log entry included.
• No documentation update required.

[hjoliver]

(Not a blocker for 8.0 obviously, but it removes some crud and should be a trivial review-and-merge)

[oliver-sanders]

or to your editor's stdin

Valid advice providing you s/(code|emacs)/vim.

cylc view (which weirdly defaults to edit instead of stdout)

That's long bugged me.

[hjoliver]

Tests all passing now. One review will do.

[wxtim]

• Mostly happy
• Played with locally

Asking for addition of obsoletion messages so that anyone with an older config gets a useful message.

[wxtim]

Happy with that.

Perhaps another day we could implement an additional upgrader method, something like quiet, which works the same as obsolete, but only logs at debug?

[hjoliver]

(Spurious test failure - artifact upload timed out)

[oliver-sanders]

(not worried about the changelog)