Users have much better, easier alternatives, like
DOMWindowUtils.{loadSheetUsingURIString,removeSheet}, which we use to
replace the only caller that exists in mozilla-central (the editor
element, which loads EditorOverride.css).

This allows to clean up the style system and editor. There are other
callers in comm-central, but it seems they can switch to DOMWindowUtils
trivially, as the DOMWindowUtils APIs also use the system principal and
thus they can load any URL.

I'll make sure to give them some time with the migration and/or help
out of course.

Differential Revision: https://phabricator.services.mozilla.com/D71263