Remove unused CSS class in Related MRs widget

What does this merge request do?

Removes an unused CSS class from related_merge_requests component.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Follow up to https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10484

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/10962