Remove stray include for dawncpp_traits.h

The CQ passed on the patch that removes dawncpp_traits even if we forgot
this include because the builders still had the generated header in
their build cache.

TBR=enga@chromium.org
BUG=dawn:125

Change-Id: Ie266bc32e6ebc4881f80c759435ca1e3f0d00e9c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/6221
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/tests/unittests/validation/ValidationTest.h

@@ -17,7 +17,6 @@
 
 #include "gtest/gtest.h"
 #include "dawn/dawncpp.h"
-#include "dawn/dawncpp_traits.h"
 
 namespace dawn_native {
     class Instance;