The `crossorigin="anonymous"` attribute was added to `<link
rel="preload">` elements created by the boot script because it was
needed when preloading API responses, so that the response can be read.

We don't need to use cross-origin requests when preloading stylesheets
however, and we were seeing occassional errors in Chrome due to this
(see https://github.com/hypothesis/client/issues/3987).