Remove legacy layout (layout 2013) (#35943)

We were already not compiling it and not running tests on it by default.
So it's simpler to just completely remove it.

Signed-off-by: Oriol Brufau <obrufau@igalia.com>

components/layout/display_list/mod.rs

@@ -1,41 +0,0 @@
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
-
-use malloc_size_of_derive::MallocSizeOf;
-use serde::{Deserialize, Serialize};
-
-pub use self::builder::{
-    BorderPaintingMode, DisplayListBuildState, IndexableText, StackingContextCollectionFlags,
-    StackingContextCollectionState,
-};
-pub use self::conversions::{FilterToLayout, ToLayout};
-
-mod background;
-mod border;
-mod builder;
-pub(crate) mod conversions;
-mod gradient;
-pub mod items;
-mod webrender_helpers;
-
-/// A unique ID for every stacking context.
-#[derive(Clone, Copy, Debug, Deserialize, Eq, Hash, MallocSizeOf, PartialEq, Serialize)]
-pub struct StackingContextId(
-    /// The identifier for this StackingContext, derived from the Flow's memory address
-    /// and fragment type.  As a space optimization, these are combined into a single word.
-    pub u64,
-);
-
-impl StackingContextId {
-    /// Returns the stacking context ID for the outer document/layout root.
-    #[inline]
-    pub fn root() -> StackingContextId {
-        StackingContextId(0)
-    }
-
-    pub fn next(&self) -> StackingContextId {
-        let StackingContextId(id) = *self;
-        StackingContextId(id + 1)
-    }
-}