When we first introduced the `discourse-details` plugin, the `<details>`
element was not supported in all browsers, so this `display: none`-based
polyfill had to be used.
Nowadays, there is no need for this. And in fact, the `details[open] >
summary:first-of-type ~ *` selector is showing as our most expensive CSS
selector during repaints.
Removing this `display: none` also means that browser find-in-page will
now be able to correctly find content inside `<details>` in discourse.
