layout: Remove the obsolete layout tracing functionality (#35001)

There were two kinds of layout tracing controlled by the same debugging
option:

 - modern layout: Functionality that dumped a JSON serialization of the
   layout tree before and after layout.
 - legacy layout: A scope based tracing that reported the process of
   layout in a structured way.

I don't think anyone working on layout is using either of these two
features. For modern layout requiring data structure to implement
`serde` serialization is incredibly inconvenient and also generates a
lot of extra code.

We also have a more modern tracing functionality based on perfetto that
we have started to use for layout and IMO it's actually being used and
more robust.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/layout/traversal.rs

@@ -224,11 +224,7 @@ fn construct_flows_at<'dom>(context: &LayoutContext, node: impl LayoutNode<'dom>
             let mut flow_constructor = FlowConstructor::new(context);
             if nonincremental_layout || !flow_constructor.repair_if_possible(&tnode) {
                 flow_constructor.process(&tnode);
-                debug!(
-                    "Constructed flow for {:?}: {:x}",
-                    tnode,
-                    tnode.flow_debug_id()
-                );
+                debug!("Constructed flow for {:?}", tnode,);
             }
         }