Backport PR #6134: remove rackcdn https workaround for mathjax cdn

cdn.mathjax.org works over https now

IPython/html/notebookapp.py

@@ -516,13 +516,7 b' class NotebookApp(BaseIPythonApplication):'
                 return url
         
         # no local mathjax, serve from CDN
-        if self.certfile:
-            # HTTPS: load from Rackspace CDN, because SSL certificate requires it
-            host = u"https://c328740.ssl.cf1.rackcdn.com"
-        else:
-            host = u"http://cdn.mathjax.org"
-        
-        url = host + u"/mathjax/latest/MathJax.js"
+        url = u"//cdn.mathjax.org/mathjax/latest/MathJax.js"
         self.log.info("Using MathJax from CDN: %s", url)
         return url