When a user clicks inside a Vue textarea (either new comment or editing
existing), we create a new `.dz-hidden-input` when Dropzone is
initialized. However, if `Cancel` or `Confirm` is clicked, those
elements remain.

To fix this, we call Dropzone's `destroy()` method to clean up these
hidden elements.

Closes https://gitlab.com/gitlab-org/gitlab/issues/209026