Fix preloading of CSS is chromium

Currently the main.css was loaded twice in Chromium because the "credentials mode" doesn't match.

 preload for 'https://shivering-isles.com/assets/css/main.css?1592603341229139384' is found, but is not used because the request credentials mode does not match. Consider taking a look at crossorigin attribute.

What it means is that resources that are fetched with crossorigin source, can't be used like they are requested from the same source. On the other hand you have to preload fonts using the crossorigin attribute.1

No matter what, by removing the crossorigin attribute from the preload, the main.css that is prefetched will then be used properly and improves the load speed of the website, because it'll be no longer waste early bandwidth or being fetched twice.