Delete unused variable initialization code

Line 4536 in tinygltf.h file, there is unused variable initialization code like below.
```c++
    const auto pBuffer = pAttribute->buffer();
```

gcc compiler complain about that unused variable. 

I found there is no side-effect in  `pAttribute->buffer()` call and also pBuffer variable is never used. Therefore I delete that line entirely.

tiny_gltf.h

@@ -4533,7 +4533,6 @@ static bool ParseDracoExtension(Primitive *primitive, Model *model,
 
     int dracoAttributeIndex = attribute.second.Get<int>();
     const auto pAttribute = mesh->GetAttributeByUniqueId(dracoAttributeIndex);
-    const auto pBuffer = pAttribute->buffer();
     const auto componentType =
         model->accessors[primitiveAttribute->second].componentType;