[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/function_expression.h b/src/tint/lang/wgsl/sem/function_expression.h
index 2be70ff..60b5470 100644
--- a/src/tint/lang/wgsl/sem/function_expression.h
+++ b/src/tint/lang/wgsl/sem/function_expression.h
@@ -26,7 +26,7 @@
 
 /// FunctionExpression holds the semantic information for expression nodes that resolve to
 /// functions.
-class FunctionExpression : public utils::Castable<FunctionExpression, Expression> {
+class FunctionExpression : public Castable<FunctionExpression, Expression> {
   public:
     /// Constructor
     /// @param declaration the AST node