Remove design_management_copy_designs feature flag

What does this MR do?

This MR removes the design_management_copy_designs feature flag to enable #13426 (closed) globally.

• Feature flag issue: #247062 (closed)
• Feature issue: #13426 (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