Revert support for ~~gh strikeout~~

src/Markdown.js

@@ -62,9 +62,6 @@ function is_multi_line(node) {
  */
 export default class Markdown {
     constructor(input) {
-        // Support GH-style strikeout
-        input = input.replace(/~~(.*?)~~/g, '<del>$1</del>');
-
         this.input = input;
 
         const parser = new commonmark.Parser();