Revert "change default MathJax CDN"

This reverts commit 802a38b86c61f470efe9767d494142817cd0141b.

It seems that jsdeliver is not safe enough.

src/resources/vnote.ini

@@ -360,7 +360,7 @@ custom_export=
 markdownit_opt=html,break,linkify,metadata
 
 ; Location and configuration for Mathjax
-mathjax_javascript=https://cdn.jsdelivr.net/npm/[email protected]/MathJax.js?config=TeX-MML-AM_HTMLorMML
+mathjax_javascript=https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.2/MathJax.js?config=TeX-MML-AM_HTMLorMML
 
 ; Styles to be removed when copied
 ; style1,style2,style3