dossier: fix upload route of piece_justificative in repetitions

In repetitions, `form.index` doesn't make much sense. Turns out we
don't really need the index of the champ: we can just use the champ id.

app/javascript/new_design/dossiers/auto-upload-controller.js

@@ -70,7 +70,7 @@ export default class AutoUploadController {
     const attachmentRequest = {
       url: autoAttachUrl,
       type: 'PUT',
-      data: `champ_id=${champId}&blob_signed_id=${blobSignedId}`
+      data: `blob_signed_id=${blobSignedId}`
     };
     await ajax(attachmentRequest);