Revert "Merge branch '471508-duo-workflow-flag' into 'master'"

Purpose of revert

This reverts merge request !158741 (merged) since it broke master given that !157907 (merged) got merged just before it and it introduced 2 spec files that relied on the legacy start_duo_workflows FF name.

Checklist

• Create an issue to reinstate the merge request and assign it to the author of the reverted merge request.
• If the revert is to resolve a broken 'master' incident, please read through the Responsibilities of the Broken master resolution DRI.
• If the revert involves a database migration, please read through Deleting existing migrations.
• Add the appropriate labels before the MR is created. We can skip CI/CD jobs only if the labels are added before the CI/CD pipeline is created.

Milestone info

• I am reverting something in the current milestone. No changelog is needed, and I've added a ~"regression:*" label.
• I am reverting something in a different milestone. A changelog is needed, and I've removed the ~"regression:*" label.

Related issues and merge requests

Related to gitlab-org/quality/engineering-productivity/master-broken-incidents#7306 (closed).