Draft: Clean up unused overriding flags in database

What does this MR do?

This MR introduces a maintaince worker to clean up unused overriding-flags from database.

The unused overridings can be fetched by (persisted_flags - YAML_definitions), because all of the feature flag references in code must have a corresponding YAML definitions.

This frees developers from the following manual step in the Feature Flag Rollout Issue. So one step ahead to automation.

• After the flag removal is deployed, clean up the feature flag by running chatops command in #production channel

Given this cleans up unused flag in database, it doesn't affect feature/application behavior at all. At any rates, the removed flags are logged/searchable in ElasticSearch/Kibana.

Related #276170

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team