[shuffle] Remove the `utils::` namespace.

This CL strips the `utils::` namespace declaration and usages.

Bug: tint:1988
Change-Id: I860b9cae052bec9eed0879fdda6ff2a05534af0e
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/143421
Auto-Submit: Dan Sinclair <dsinclair@chromium.org>
Reviewed-by: James Price <jrprice@google.com>
Commit-Queue: David Neto <dneto@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: David Neto <dneto@google.com>
diff --git a/src/tint/lang/wgsl/sem/index_accessor_expression.h b/src/tint/lang/wgsl/sem/index_accessor_expression.h
index 3a30e0c..db1c60a 100644
--- a/src/tint/lang/wgsl/sem/index_accessor_expression.h
+++ b/src/tint/lang/wgsl/sem/index_accessor_expression.h
@@ -23,8 +23,7 @@
 namespace tint::sem {
 
 /// IndexAccessorExpression holds the semantic information for a ast::IndexAccessorExpression node.
-class IndexAccessorExpression final
-    : public utils::Castable<IndexAccessorExpression, AccessorExpression> {
+class IndexAccessorExpression final : public Castable<IndexAccessorExpression, AccessorExpression> {
   public:
     /// Constructor
     /// @param declaration the AST node