Destroy Dropzone hidden input when form is destroyed

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: https://gitlab.com/meno/dropzone/-/blob/6cef0c303a472468f8d3581c82062a6d31045666/src/dropzone.js#L1215-1224

Closes #209026 (closed)