Remove legacy changelog directory

What does this MR do?

This removes a legacy changelog directory that we have not supported in a long time. The existing entries were for 13.0 by the looks of it, which has already been released.

This fixes gitlab-com/gl-infra/delivery#884 (closed)

Does this MR meet the acceptance criteria?

Conformity

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