Bug 1839255 - Clean up a bit the mapped attribute set-up. r=smaug

This doesn't change behavior on its own. It could be split up a bit more
if needed though over all it shouldn't be hard to follow.

Unify nsHTMLStyleSheet and nsHTMLCSSStyleSheet into AttributeStyles
because the fact that we have two right now is pretty silly. They are
also not stylesheets (they used to be pre-stylo).

Differential Revision: https://phabricator.services.mozilla.com/D181414

dom/html/nsGenericHTMLElement.cpp

@@ -29,7 +29,6 @@
 #include "mozilla/dom/BindContext.h"
 #include "mozilla/dom/Document.h"
 #include "nsMappedAttributes.h"
-#include "nsHTMLStyleSheet.h"
 #include "nsPIDOMWindow.h"
 #include "nsIFrameInlines.h"
 #include "nsIScrollableFrame.h"