Remove addons field is not used anymore (#529)

Required by readthedocs/readthedocs.org#11767

Author: humitos

readthedocsext/theme/templates/projects/addons_form.html

@@ -92,7 +92,6 @@
       {% block addons_docdiff %}
         <div class="ui tab" data-tab="docdiff">
           {{ form.doc_diff_enabled | as_crispy_field }}
-          {{ form.doc_diff_root_selector | as_crispy_field }}
         </div>
       {% endblock addons_docdiff %}