Fix warnings from about unused variables that are kept there for legacy deserialization

com.unity.shadergraph/Editor/Data/Graphs/SerializableCubemap.cs

         [Serializable]
         class CubemapHelper
         {
+#pragma warning disable 649
+#pragma warning restore 649
         }
 
         public Cubemap cubemap

com.unity.shadergraph/Editor/Data/Graphs/SerializableMesh.cs

         [Serializable]
         class MeshHelper
         {
+#pragma warning disable 649
+#pragma warning restore 649
         }
 
         public Mesh mesh

com.unity.shadergraph/Editor/Data/Graphs/SerializableTexture.cs

         [Serializable]
         class TextureHelper
         {
+#pragma warning disable 649
+#pragma warning restore 649
         }
 
         public Texture texture