Remove recursive_approach_for_all_projects FF [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Part of #263442 (closed)

The recursive_approach_for_all_projects feature flag was progressively rolled out on production in the span of a week and has been fully enabled for a few days now (since 2020-02-19). With the successful rollout and the feature flag being enabled for a while now it safe to remove the feature flag from the code. This merge request will remove it and add the required changelog for the change.

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