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/construct.rs

@@ -97,14 +97,6 @@ impl ConstructionResult {
         // inefficient!
         (*self).clone()
     }
-
-    pub fn debug_id(&self) -> usize {
-        match *self {
-            ConstructionResult::None => 0,
-            ConstructionResult::ConstructionItem(_) => 0,
-            ConstructionResult::Flow(ref flow_ref, _) => flow_ref.base().debug_id(),
-        }
-    }
 }
 
 /// Represents the output of flow construction for a DOM node that has not yet resulted in a