remove old code

docs/_static/custom_collapsible_code.css

@@ -1,17 +0,0 @@
-.toggle {
-    padding-bottom: 1em ;
-}
-
-.toggle .header {
-    display: block;
-    clear: both;
-    cursor: pointer;
-}
-
-.toggle .header:after {
-    content: " ▼";
-}
-
-.toggle .header.open:after {
-    content: " ▲";
-}

docs/_static/custom_collapsible_code.js

@@ -1,10 +0,0 @@
-let makeCodeBlocksCollapsible = function() {
-    $(".toggle > *").hide();
-    $(".toggle .header").show();
-    $(".toggle .header").click(function() {
-        $(this).parent().children().not(".header").toggle({"duration": 400});
-        $(this).parent().children(".header").toggleClass("open");
-    });
-};
-// we could use the }(); way if we would have access to jQuery in HEAD, i.e. we would need to force the theme
-// to load jQuery before our custom scripts