Use the standard way of detecting RTD

docs/_templates/layout.html

@@ -27,7 +27,7 @@
   </div>
   {% endif %}
 
-  {%- if ON_READTHEDOCS %}
+  {%- if READTHEDOCS %}
   <script>
     if (typeof READTHEDOCS_DATA !== 'undefined') {
         if (!READTHEDOCS_DATA.features) {

docs/conf.py

@@ -143,10 +143,6 @@ html_experimental_html5_writer = True
 # a list of builtin themes.
 html_theme = 'basic'
 
-html_context = {
-  'ON_READTHEDOCS': bool(os.environ.get('READTHEDOCS'))
-}
-
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the
 # documentation.