Make a cleanup after Design Management feature flag is removed

What does this MR do?

This MR is a part of the work for moving Design Management app to the Issue description. In this part, we remove the feature flag and clean up all the logic related to it

Change	MR
Create a new Design Management app to be displayed behind the feature flag	!34932 (merged)
Refactor Design Management app to be displayed on the Issue instead of tabs	!35042 (merged)
Refactor Design management application to match new visual styles	!35746 (merged)
Update Design Management documentation	!36681 (merged)
Make a cleanup after Design Management feature flag is removed	👉🏻 You are here

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides

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

Closes #223197 (closed)