Remove Empy Support

[wxtim]

Cylc will no longer support Empy from 8.4.0. This commit removes empy functionality and tests from Cylc Rose.

Follow on from cylc/cylc-flow#6476

n.b. Functional Changes in bcf1629. Other commit should just be styling things.

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.

[markgrahamdawson]

If I look for this cylc_flow_opts variable in cylc-rose code it only exists in the yaml file. Is it needed?

[wxtim]

This is an argument to the cylc-release-actions action. On looking, it's not required, so is probably unecessary. Will try removal.

[oliver-sanders]

The default is 'all' (equivalent to pip install cylc-flow[all]) which is unnecessary for these tests, leaving it as '' (as you have done) is the correct thing to do.