Remove polyfill JavaScript.

Author: lawmurray

src/doxide.cpp

@@ -52,7 +52,6 @@ R""""(site_name:
   - stylesheets/doxide.css
 extra_javascript:
   - javascripts/mathjax.js
-  - https://polyfill.io/v3/polyfill.min.js?features=es6
   - https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js
 )"""";