Remove vim.lang from syntax

[wxtim]

Point users of Cylc Get Resources to the new cylc.vim plugin.

closes #5626 .

Is probably massive overkill.

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).
• Small change to peripheral script.
• Changelog entry included if this is a change that can affect users
• Self documenting
• Deprecation, raised against master