refactor: retire GCObject.shallow_copy()

Collector gets this info from gc-owned object header

include/xo/object2/DStruct.hpp

@@ -141,8 +141,6 @@ namespace xo {
             ///@{
             /** shallow memory consumption. Excludes child objects **/
             AAllocator::size_type shallow_size() const noexcept;
-            /** return shallow copy of this array, using memory from @p mm **/
-            DStruct * shallow_copy(obj<AAllocator> mm) const noexcept;
             /** forward elements to @p gc to-space; replace originals with forarding pointers **/
             AAllocator::size_type forward_children(obj<ACollector> gc) noexcept;
             ///@}