Deleting the last comment at a given placeholder should remove the underlining

[sampaccoud]

Bug Report

Problematic behavior
The underlining remains even after the last comment was deleted

Expected behavior/code
The underlining should disappear

Steps to Reproduce

1. Go to a document
2. Create a comment
3. Delete the comment
4. The underlining remains

Environment

• Docs version: 4.0.0
• Instance url: docs.numerique.gouv.fr

Possible Solution
Detect when the comment being deleted is the last one and clean the placeholder.

[sampaccoud]

oups. Fixed here #1687