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_2020/fragment_tree/hoisted_shared_fragment.rs

@@ -3,7 +3,6 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use app_units::Au;
-use serde::Serialize;
 use style::logical_geometry::WritingMode;
 use style::values::specified::align::AlignFlags;
 
@@ -13,7 +12,6 @@ use crate::geom::{LogicalVec2, PhysicalRect, PhysicalVec};
 /// A reference to a Fragment which is shared between `HoistedAbsolutelyPositionedBox`
 /// and its placeholder `AbsoluteOrFixedPositionedFragment` in the original tree position.
 /// This will be used later in order to paint this hoisted box in tree order.
-#[derive(Serialize)]
 pub(crate) struct HoistedSharedFragment {
     pub fragment: Option<Fragment>,
     /// The "static-position rect" of this absolutely positioned box. This is defined by the