layout: Remove the now-useless dirty field from the layout context.

At this point the only thing it does is to try to avoid adding display
items that are outside the root scrollable area, which is both wrong
(since it's incompatible with having scrollable areas outside the root)
and is useless (because we have displayports now).

components/layout/context.rs

@@ -109,9 +109,6 @@ pub struct SharedLayoutContext {
     /// The URL.
     pub url: Url,
 
-    /// The dirty rectangle, used during display list building.
-    pub dirty: Rect<Au>,
-
     /// Starts at zero, and increased by one every time a layout completes.
     /// This can be used to easily check for invalid stale data.
     pub generation: u32,