javascriptguide: turn down old ES5 guide

We kept this around for posterity, but all things die eventually.
ES6 was released over a decade ago and there's really no reason to
write new code targeting ES5 or older.  If you actually need to
target such a system, transpilers like Babel are the way to go.
Removing this guide allows us to remove the styleguide.css file as
no other guide relies on it.

404.html

@@ -15,7 +15,8 @@ window.onload = function() {
     'cppguide.xml': 'cppguide.html',
     'google-r-style.html': 'Rguide.html',
     'htmlcssguide.xml': 'htmlcssguide.html',
-    'javascriptguide.xml': 'javascriptguide.html',
+    'javascriptguide.xml': 'jsguide.html',
+    'javascriptguide.html': 'jsguide.html',
     'jsoncstyleguide.html': 'jsoncguide.html',
     'jsoncstyleguide.xml': 'jsoncguide.html',
     'lispguide.xml': 'lispguide.html',