The page is using jQuery, which, by default, defines $ for "selecting" elements as well.
You can use the longer document.querySelector(selector) in pages like that page.
If you need the second parameter ($("link", document.head)), you can just use -
secondParameter.querySelector(selector). For example, document.head.querySelector("link").
Alternatively - use $$, which returns a list and you can use [0] to get the first. For example -
$$("link")[0]
Or -
$$("link", document.head)[0]
$("link")[0] will also work.
Finally, note that deleting $ first will also work (delete $;), but may break the page.