Remove polyfill.io link from head custom (#9678)

Signed-off-by: Fanit Kolchina <[email protected]>
(cherry picked from commit 3697333)
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

_includes/head_custom.html

@@ -3,7 +3,6 @@
 {% endif %}
 
 {% if page.has_math == true %}
-  <script src="https://polyfill.io/v3/polyfill.min.js?features=es6"></script>
   <script id="MathJax-script" async src="https://cdn.jsdelivr.net/npm/[email protected]/es5/tex-mml-chtml.js"></script>
 {% endif %}