Remove used member 'm_is_built'

This is now computed through m_connectivity_ptr status
Also changed item_t from const to constexpr

src/mesh/ItemValue.hpp

@@ -18,7 +18,7 @@ template <typename DataType,
 class ItemValue
 {
  public:
-  static const ItemType item_t{item_type};
+  static constexpr ItemType item_t{item_type};
   using data_type = DataType;
 
   using ItemId = ItemIdT<item_type>;
@@ -33,8 +33,6 @@ class ItemValue
 
   ConnectivityPtr m_connectivity_ptr;
 
-  bool m_is_built{false};
-
   Array<DataType> m_values;
 
   // Allow const std:shared_ptr version to access our data