Remove ci_destroy_all_expired_service feature flag

What does this MR do and why?

This merge request removes the ci_destroy_all_expired_service feature flag. It was enabled by default in 14.9 and the enabled-by-default setting has also been backported in a patch version all the way back to 14.6. All versions of GitLab are removing expired artifacts at some pace, and we're confident in the operational stability and correctness of that process.

There's some discussion of a generalized way to turn workers on and off in the way that we used this flag, but as the consensus is that the switch shouldn't live in the application's business logic, we agree that this particular flag can and should be removed.

Screenshots or screen recordings

How to set up and validate locally

1. This flag is already enabled by default, so all development, test, and production environments are already performing the behavior that we enable with this flag.

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.

Closes #348786 (closed)