Remove DAWN_USE_NEW_TINT_TRANSFORM_API

The legacy code path has now been removed from dawn:
https://dawn-review.googlesource.com/c/dawn/+/34900

Bug: tint:390
Bug: tint:389
Change-Id: Id897bae3d29c7ff5bb0e2e32db1a4b493d4fe715
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/35004
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
diff --git a/src/transform/manager.h b/src/transform/manager.h
index 4d8662a..b31a81c 100644
--- a/src/transform/manager.h
+++ b/src/transform/manager.h
@@ -23,11 +23,6 @@
 #include "src/diagnostic/diagnostic.h"
 #include "src/transform/transform.h"
 
-// A define used by Dawn to atomically switch to the new tint::transform API
-// when the API breaking change lands.
-// TODO(bclayton) - Remove once migration is complete
-#define DAWN_USE_NEW_TINT_TRANSFORM_API 1
-
 namespace tint {
 namespace transform {