Remove optimized_elasticsearch_indexes_project FF

What does this MR do?

This change removes the optimized_elasticsearch_indexes_project feature flag. The feature flag was enabled by default and I didn't find any issues on GL.com (sentry).

The original method (elasticsearch_limited_projects) is still in the codebase, as far as I can tell a rake task (ee/lib/tasks/gitlab/elastic.rake) is referencing it. Note: the optimized query only works for one project, however elasticsearch_limited_projects is able to return all projects.

Screenshots

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