Stop posting file_id when sending a letter

The endpoint works fine with it in the URL now instead, so we need stop
posting it. We can’t stop expecting it yet, because some old instances
will still be posting to the endpoint without the ID in the url.

app/templates/views/uploads/preview.html

@@ -55,9 +55,9 @@
         <form method="post" enctype="multipart/form-data" action="{{url_for(
             'main.send_uploaded_letter',
             service_id=current_service.id,
+            file_id=file_id,
           )}}" class='page-footer'>
             {{ radios(form.postage, hide_legend=true, inline=True) }}
-            {{ form.file_id(value=file_id) }}
             {{ page_footer("Send 1 letter") }}
         </form>
       {% endif %}