Drafts can't be discarded in merge request comments

Summary

When I write a comment in a merge request diff and discard the changes, the comment draft is not deleted.

Steps to reproduce

1. Open a merge request
2. Navigate to the "changes" tab
3. Add a comment to a file.
4. Write something in the Content Editor or the Markdown Editor.
5. Select "Cancel" to discard the draft.
6. Repeat steps 3 and 4 on the same line selected previously.
7. You will notice that the draft was not discarded.

Example Project

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

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(For installations with omnibus-gitlab package run and paste the output of: \`sudo gitlab-rake gitlab:env:info\`) (For installations from source run and paste the output of: \`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production\`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of: `sudo gitlab-rake gitlab:check SANITIZE=true`)

(For installations from source run and paste the output of: `sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true`)

(we will only investigate if the tests are passing)

Possible fixes