[Feature flag] Cleanup of `ci_instance_variables_ui`

What

The feature flag ci_instance_variables_ui has been introduced in %13.1 and enabled by default for that long.

Code that uses ci_instance_variables_ui should be cleaned up as if the flag was enabled by default.

Owners

• Team: ~"group::continuous integration"
• Most appropriate slack channel to reach out to: #g_ci
• Best individual to reach out to: @pburdette

Expectations

What are we expecting to happen?

What might happen if this goes wrong?

What can we monitor to detect problems with this?

Beta groups/projects

If applicable, any groups/projects that are happy to have this feature turned on early. Some organizations may wish to test big changes they are interested in with a small subset of users ahead of time for example.

• gitlab-org/gitlab project
• gitlab-org/gitlab-com groups
• ...

Roll Out Steps

• Remove feature flag and add changelog entry
• clean up the feature flag by running chatops command in #production channel

Rollback Steps

• This feature can be disabled by running the following Chatops command:

/chatops run feature set --project=gitlab-org/gitlab feature_name false