Remove dead a/b testing (sixpack) code

[zackkrida]

Fixes

Fixes #789 by @zackkrida

Description

Removes all code related to the sixpack (a/b testing) module.

Testing Instructions

Observe the tests continue to pass and the application works locally as expected.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the default branch of the repository (main) or a parent feature branch.
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.

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[sarayourfriend]

Reprioritizing to critical vis-a-vis potential GDPR cookie non-compliance.

[sarayourfriend]

Looks good so far, but we should also remove the code here that adds the abSessionId cookie.

[zackkrida]

Such an obvious omission, thanks Sara!

[sarayourfriend]

LGTM!

[krysal]

LGTM