Remove polyfill. (#620)

The `polyfill.io` domain was taken over by a malicious actor, and now
injects code that redirects to unrelated websites.

References:

<https://sansec.io/research/polyfill-supply-chain-attack>
<https://blog.cloudflare.com/automatically-replacing-polyfill-io-links-with-cloudflares-mirror-for-a-safer-internet/>

Reported-By: Elliott Green <eg145@brighton.ac.uk>

_includes/head.html

@@ -43,7 +43,6 @@
   {%- feed_meta -%}
   <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.1/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-4bw+/aepP/YC94hEpVNVgiZdgIC5+VKNBQNGCHeKRQN+PtmoHDEXuppvnDJzQIu9" crossorigin="anonymous">
   <link rel="stylesheet" href="{{ '/assets/css/override.css' | relative_url }}" />
-  <script src="https://polyfill.io/v3/polyfill.min.js?features=es6"></script>
   <script src="https://cdn.jsdelivr.net/npm/@popperjs/core@2.11.6/dist/umd/popper.min.js" integrity="sha384-oBqDVmMz9ATKxIep9tiCxS/Z9fNfEXiDAYTujMAeBAsjFuCZSmKbSSUnQlmh/jp3" crossorigin="anonymous"></script>
   <script src="https://cdn.jsdelivr.net/npm/bootstrap@5.3.1/dist/js/bootstrap.min.js" integrity="sha384-Rx+T1VzGupg4BHQYs2gCW9It+akI2MM/mndMCy36UVfodzcJcF0GGLxZIzObiEfa" crossorigin="anonymous"></script>
   <script type="text/javascript">