layout: Remove IndependentNonReplacedContents (#37971)

With the work to unify the layout logic for replaced and non-replaced
boxes (#37942), I think the `IndependentNonReplacedContents` enum does
no longer make much sense.

Therefore, this removes `IndependentNonReplacedContents`, merging its
values into `IndependentFormattingContextContents`.

The methods defined on `IndependentFormattingContextContents` can now be
on `IndependentFormattingContext`, in particular this implies that the
layout results of a replaced box will now be cached.

Testing: Unneeded (no behavior change)
This part of #37942

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

components/layout/replaced.rs

@@ -585,7 +585,6 @@ impl ReplacedContents {
         depends_on_block_constraints: bool,
         lazy_block_size: &LazySize,
     ) -> CacheableLayoutResult {
-        // TODO: consider caching the result in LayoutBoxBase like we do for non-replaced.
         let writing_mode = base.style.writing_mode;
         let inline_size = containing_block_for_children.size.inline;
         let content_block_size = self.content_size(