DOC/BLD: remove polyfill

It seems `polyfill` is no longer required per the docs:
https://squidfunk.github.io/mkdocs-material/reference/math/#mathjax-mkdocsyml
And perhaps also a security concern.

mkdocs.yml

@@ -58,7 +58,6 @@ markdown_extensions:
 
 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      
 
 extra: