[WebNN EP] Remove useless variable unpacked_tensors_ (microsoft#21189)

Author: Honry

onnxruntime/core/providers/webnn/builders/model_builder.cc

@@ -104,8 +104,7 @@ Status ModelBuilder::RegisterInitializers() {
       if (tensor.has_raw_data()) {
         tensor_ptr = reinterpret_cast<std::byte*>(const_cast<char*>(tensor.raw_data().c_str()));
       } else {
-        unpacked_tensors_.push_back({});
-        std::vector<uint8_t>& unpacked_tensor = unpacked_tensors_.back();
+        std::vector<uint8_t> unpacked_tensor;
         ORT_RETURN_IF_ERROR(onnxruntime::utils::UnpackInitializerData(tensor, unpacked_tensor));
         tensor_ptr = reinterpret_cast<std::byte*>(unpacked_tensor.data());
       }

onnxruntime/core/providers/webnn/builders/model_builder.h

@@ -66,7 +66,6 @@ class ModelBuilder {
   emscripten::val wnn_builder_ = emscripten::val::object();
   DataLayout preferred_layout_;
   WebnnDeviceType wnn_device_type_;
-  std::vector<std::vector<uint8_t>> unpacked_tensors_;
   InlinedHashMap<std::string, emscripten::val> wnn_operands_;
   std::vector<std::string> input_names_;
   std::vector<std::string> output_names_;