Security: Remove malicious polyfill.io script due to supply-chain attack

The polyfill.io domain was compromised in a major supply-chain malware attack. 
This script is no longer necessary as modern browsers natively support ES6 features.

Removes the line:
<script defer src="https://polyfill.io/v3/polyfill.min.js?features=es6"></script>

Author: JLPM22

docs/_includes/scripts/mathjax.html

@@ -1,4 +1,4 @@
-  {%- if site.enable_math -%}
+{%- if site.enable_math -%}
   <!-- MathJax -->
   <script type="text/javascript">
     window.MathJax = {
@@ -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 %}