Remove useless "early render" in authorize.php

This is part of #2046881: [meta] Avoid "early rendered" strings where beneficial to do so, build structured data to pass to drupal_render() once instead.

Line 147 of authorize.php:

$output = drupal_render($elements);

This $output is only used in the following:

print theme('maintenance_page', array('content' => $output, 'show_messages' => $show_messages));

maintenance-page.html.twig does this:

{{ content }}

This means the initial render of $output is useless and we should be able to safely turn this:

      // We have a batch to process, show the filetransfer form.
      $elements = drupal_get_form('authorize_filetransfer_form');
      $output = drupal_render($elements);

into:

      // We have a batch to process, show the filetransfer form.
      $output = drupal_get_form('authorize_filetransfer_form');