Cleanup `ci_remove_legacy_predefined_variables`

What does this MR do and why?

This MR cleans up the FF ci_remove_legacy_predefined_variables which was defaulted to ON in %16.0 in #404533 (closed).

This feature removes the legacy predefined CI_BUILD_* variables, which was announced as a breaking change for %16.0: https://docs.gitlab.com/ee/update/removals.html#ci_build_-predefined-variables-removed

Resolves #410235 (closed)

How to set up and validate locally

1. Check out this branch.
2. Update your project's .gitlab-ci.yml with: gitlab-ci.yml
3. Run the pipeline and play the manual job. Check the job log. Ensure that the legacy variables do not have any value.

Screenshot

MR acceptance checklist

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Related to #410235 (closed)