Remove `design_management_design_notification_participants`"

What does this MR do?

This MR removes the design_management_design_notification_participants feature flag, enabling the bugfix #217095 (closed) for everyone.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry The bugfix MR accidentally included the changelog, even though it also included the feature flag !46642 (diffs) 🤦
• 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

Related to #277319 (closed) #217095 (closed)