LibWeb: Remove WidgetBox layout node

The approach of attaching sub-widgets to the web view widget was only
ever going to work in single-process mode, and that's not what we're
about anymore, so let's just get rid of WidgetBox so we don't have the
dead-end architecture hanging over us.

The next step here is to re-implement <input type=text> using LibWeb
primitives.

Userland/Libraries/LibWeb/InProcessWebView.cpp

@@ -401,7 +401,6 @@ void InProcessWebView::set_document(DOM::Document* document)
 void InProcessWebView::did_scroll()
 {
     page().main_frame().set_viewport_scroll_offset({ horizontal_scrollbar().value(), vertical_scrollbar().value() });
-    page().main_frame().did_scroll({});
 }
 
 void InProcessWebView::drop_event(GUI::DropEvent& event)