Remove vertical connecting line placeholder from diff discussion notes
What does this MR do?
Removes the vertical connecting line in diff discussions. We used to have diff notes in boxes with connecting lines (like the discussion tab of an issue/MR) but we ended up removing those to add more space for the actual notes. This line is a holdout from that.
See gif in issue description https://gitlab.com/gitlab-org/gitlab-ce/issues/57768
Closes #57768 (closed)