Vue markdown fields leave `.dz-hidden-input` in DOM

From #205408 (comment 298684000)

When we click inside a Vue textarea (either new comment or editing existing), we create a new .dz-hidden-input. When we Confirm/Cancel we don't remove the .dz-hidden-input element.

To reproduce:

1. (in console) run document.querySelectorAll('.dz-hidden-input').length - probably prints 1
2. Click in a comment area (either reply or Add comment)
3. Rerun command from step 1, now prints 2
4. Click outside the textarea (cancel comment) then back in, now there are 3!