Removed Read the Docs version overlay

These HTML template changes make Read the Docs believe that there
is already a version overlay in the HTML documentation, so it does
not inject its own overlay when building the docs on their server.

Author: kbg

doc/_templates/layout.html

@@ -7,3 +7,8 @@
     </script>
     {% endif %}
 {% endblock %}
+
+{%- block relbar2 %}
+{{ relbar() }}
+{% include "versions.html" %}
+{% endblock %}

doc/_templates/versions.html

@@ -0,0 +1,6 @@
+{% if READTHEDOCS %}
+  <div class="rst-versions" data-toggle="rst-versions" role="note" aria-label="versions" style="display: none;">
+    <div class="rst-other-versions" style="display: none;">
+    </div>
+  </div>
+{% endif %}