removed polyfill.io

Author: stoyanovich

_includes/scripts/mathjax.html

@@ -8,5 +8,4 @@
     };
   </script>
   <script defer type="text/javascript" id="MathJax-script" src="https://cdn.jsdelivr.net/npm/mathjax@{{ site.mathjax.version }}/es5/tex-mml-chtml.js"></script>
-  <script defer src="https://polyfill.io/v3/polyfill.min.js?features=es6"></script>
   {%- endif %}