Don't try to deduplicate scheduled jobs

What does this MR do?

This avoids marking jobs that are scheduled in the future as duplicates when there's already a job in the queue with the same arguments. They're marked in the future, so it's likely we intend to run it again after a while.

Required for gitlab-com/gl-infra/scalability#241 (closed)

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