attachments_ui: Remove unused 'id' from 'Delete file' dialog widget.

The 'id' field in a dialog widget is used to add custom id to
the container element to modify styles.

We were not using this id anywhere, so this commit removes it.

web/src/attachments_ui.ts

@@ -92,7 +92,6 @@ function delete_attachments(attachment: string, file_name: string): void {
         html_heading: $t_html({defaultMessage: "Delete file?"}),
         html_body,
         html_submit_button: $t_html({defaultMessage: "Delete"}),
-        id: "confirm_delete_file_modal",
         focus_submit_on_open: true,
         on_click() {
             dialog_widget.submit_api_request(channel.del, "/json/attachments/" + attachment, {});