Remove design_management_todos_api feature flag

What does this MR do?

This MR removes the design_management_todos_api feature flag.

The feature flag has been enabled by default for a number of releases.

A changelog has not been added, as it was added when the feature flag was enabled by default 2b06001c.

The feature flag has been confirmed as having been removed from the feature flipping database in production.

Issue: #32492 (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.