Remove feature flag for dropping duplicate jobs

What does this MR do?

This removes the feature flag for gitlab-com/gl-infra/scalability#218 (closed) and gitlab-com/gl-infra/scalability#42 (closed)

The feature has been enabled on GitLab.com since gitlab-com/gl-infra/scalability#42 (comment 318078109)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [x-] 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