#2118 - Remove CLEAR_CELERY_CHAIN Feature Flag

[k-macmillan]

Description

This is to clean up the Feature Flag implemented in #2053 . This sat around for a bit longer than planned due to some odd behavior we saw on staging that we were unable to replicate, track down a source, or tie to 2053.

issue #2118

How Has This Been Tested?

This has been sitting on all lowers for nearly a week without any issues tied to it. We're considering this safe for prod now.

Deployed to Dev, regressions passed.

Checklist

• [x] I have assigned myself to this PR
• [x] PR has an appropriate title: #9999 - What the thing does
• [x] PR has a detailed description, including links to specific documentation
• [x] I have added the appropriate labels to the PR.
• [x] I did not remove any parts of the template, such as checkboxes even if they are not used
• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to any documentation
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works. Testing guidelines
• [x] I have ensured the latest main is merged into my branch and all checks are green prior to review
• [x] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules
• [x] The ticket was moved into the DEV test column when I began testing this change