Revert "Merge branch '468827-optimize-ListConfigVariablesService' into 'master'"

Purpose of revert

Reverted MR: Optimize ListConfigVariablesService (prefill va... (!162055 - merged)

gitlab-com/gl-infra/production#18401 (closed)

• https://gitlab.com/gitlab-org/gitlab/-/issues/479293+
• https://gitlab.com/gitlab-org/gitlab/-/issues/479294+
• https://gitlab.com/gitlab-org/gitlab/-/issues/479295+.

I tested the prefill variables and it does not work on Staging.

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