docs: remove tracking through Google Analytics (#6843)

This might not even be monitored by anyone, but if it is useful in
general should probably get replaced with a less controversial provider.

doc/en/_templates/layout.html

@@ -1,20 +0,0 @@
-{% extends "!layout.html" %}
-{% block header %}
-  {{super()}}
-{% endblock %}
-{% block footer %}
-{{ super() }}
-<script type="text/javascript">
-
-  var _gaq = _gaq || [];
-  _gaq.push(['_setAccount', 'UA-7597274-13']);
-  _gaq.push(['_trackPageview']);
-
-  (function() {
-    var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
-    ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
-    var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
-  })();
-
-</script>
-{% endblock %}