Remove Hypothesis plugin

We don't use it, it sets cookies we'd otherwise have to disable and
probably collects information we'd have to explain in our privacy
policy. It's altogether easier to just get rid of it.

docs/_templates/layout.html

@@ -1,6 +1,5 @@
 {% extends "!layout.html" %}
 {% block footer %}
-<script async defer src="https://hypothes.is/embed.js"></script>
 
 <!-- Global site tag (gtag.js) - Google Analytics -->
 <script async src="https://www.googletagmanager.com/gtag/js?id=UA-158633237-1"></script>