Remove unused variable

This blocks the import into google3.

Change-Id: Ieb5db932553b5f9c8e44e1c334e497004c6bb778
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/116361
Commit-Queue: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Austin Eng <enga@chromium.org>

src/dawn/native/ShaderModule.cpp

@@ -529,7 +529,6 @@ ResultOrError<std::unique_ptr<EntryPointMetadata>> ReflectEntryPointUsingTint(
 
     if (!entryPoint.overrides.empty()) {
         const auto& name2Id = inspector->GetNamedOverrideIds();
-        const auto& id2Scalar = inspector->GetOverrideDefaultValues();
 
         for (auto& c : entryPoint.overrides) {
             auto id = name2Id.at(c.name);