Mark the ExpirePipelineCacheWorker as idempotent

What does this MR do?

This worker expires cache, so it is already idempotent: expiring the cache twice does nothing.

This will allow us to deduplicate this queue.

For gitlab-com/gl-infra/scalability#284 (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

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