move code cleaning cylc-rose opts to cylc-rose post install plugin

[wxtim]

Move clearing of Cylc Rose options from Cylc to rose - companion PR of cylc/cylc-rose#308

Should be applied to master instead of #6059

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 Should be manually tested using examples in Changed behaviour in handling template variables passed via cli #6058 and Clear Template Variables from scheduler options before reload #5996
• CHANGES.md 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.

[wxtim]

I don't think that this will work. Explanation on cylc/cylc-rose#308 (comment)