Remove design_management_flag feature flag

What does this MR do?

This MR removes the design_management_flag feature flag from our codebase.

• Issue #13709 (closed)

A changelog entry does not need to be added, as the feature has been enabled by default and has already been announced.

The design_management_flag flag itself was deleted in 12.2

/chatops run feature get design_management_flag
=> The feature "design_management_flag" does not exist.

Screenshots

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