Remove deprecated design route

What does this MR do?

This MR removed a deprecated Design Management route.

The route had been deprecated in %12.9 in !25781 (merged), where the route became a redirect to the new route.

The route is used to serve a full-size design file. Users never visit this route themselves. The deprecation was put in place on the chance that someone had cached the old URIs (exposed in GraphQL API) and were continuing to use those old URIs for some reason.

I've searched Kibana for the deprecated route, and can't see any use of those routes on GitLab.com, so I believe it's safe to go ahead and remove these in %13.0.

• Issue: #208256 (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