Remove accidental unused Transform Manager

Removes an unused tint::Transform::Manager that was accidentally merged
as part of commit "Integrate Multiplanar External Texture Transform".

Bug: dawn:1082
Change-Id: I55b5eec8b17f27df22658a90912dec4d6d583cbb
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/80100
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Brandon1 Jones <brandon1.jones@intel.com>

src/dawn/native/ShaderModule.cpp

@@ -1307,7 +1307,6 @@ namespace dawn::native {
                     {static_cast<uint32_t>(i), static_cast<uint32_t>(expansion.second.params)}};
             }
         }
-        tint::transform::Manager m;
 
         if (!newBindingsMap.empty()) {
             transformManager->Add<tint::transform::MultiplanarExternalTexture>();