Remove attempt_project_search_optimizations feature flag

What does this MR do?

Related to #214382 (closed), removing the attempt_project_search_optimizations feature flag.

As discussion at !40881 (comment 406330839), only remove feature flag but keep the parameter.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
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• 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

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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