Clean up all references of `ff_ones_issues_integration`

What does this MR do and why?

I found this accidentally when investigating https://jihulab.com/jihulab/customersuccess/customer-success-teamwork/-/issues/5783 (internal only and isn't really related to this MR), it sounds like we removed the ff_ones_issues_integration feature flag shortly after the docs were added and both commits are first presented in v15.5.0-jh (according to "tags containing commit"). I guess it should be possible to not mention this feature flag in docs at all since 15.4 (or older) users shouldn't aware of this flag, while 15.5 (or later) users should enjoy the feature without enabling the flag.

While at it, I am removing all references of ff_ones_issues_integration (in the e2e tests), assuming they are no-op. I honestly didn't try to run the tests myself so hopefully pipelines can catch the issues for me, this is why I am drafting this MR :-)

Screenshots or screen recordings

I don't aware of any UI changes in this MR! But anyway here is a screenshot for 15.5.9-jh with ONES integration configured.

How to set up and validate locally

• Prepare an gitlab-jh installation (15.5 or later).
• Make sure ff_ones_issues_integration is not presented in Feature.all (related docs).
• Try to configure ONES integration.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.