DEV: Remove the <details> polyfill (PR #10260)

Per the linked site (http://mths.be/details) and https://caniuse.com/#feat=details the <details> element is supported since Chrome 12/Safari 6/Firefox 49/Edge 79. IE 11 doesn’t handle it but we dropped the support for that browser.

Removing this fixes the lint issues uncovered in https://github.com/discourse/discourse/pull/10259.

GitHub

Less code is always better code :+1: