refactor: retire GCObject.shallow_size()

Not needed.  Rely on size stored in gc-owned object header

include/xo/alloc2/gc/AGCObject.hpp

@@ -62,8 +62,6 @@ public:
     virtual typeseq _typeseq() const noexcept = 0;
     /** destroy instance @p d; calls c++ dtor only for actual runtime type; does not recover memory **/
     virtual void _drop(Opaque d) const noexcept = 0;
-    /** memory consumption for this instance **/
-    virtual size_type shallow_size(Copaque data)  const  noexcept = 0;
 
     // nonconst methods
     /** move instance using allocator **/

include/xo/alloc2/gc/IGCObject_Any.hpp

@@ -61,7 +61,6 @@ namespace mm {
         [[noreturn]] void _drop(Opaque) const noexcept override { _fatal(); }
 
         // const methods
-        [[noreturn]] size_type shallow_size(Copaque)  const  noexcept override { _fatal(); }
 
         // nonconst methods
         [[noreturn]] Opaque shallow_move(Opaque, obj<ACollector>)  const  noexcept override;

include/xo/alloc2/gc/IGCObject_Xfer.hpp

@@ -49,9 +49,6 @@ namespace mm {
         void _drop(Opaque d) const noexcept override { _dcast(d).~DRepr(); }
 
         // const methods
-        size_type shallow_size(Copaque data)  const  noexcept override {
-            return I::shallow_size(_dcast(data));
-        }
 
         // non-const methods
         Opaque shallow_move(Opaque data, obj<ACollector> gc)  const  noexcept override {

include/xo/alloc2/gc/RGCObject.hpp

@@ -55,9 +55,6 @@ public:
     void _drop() const noexcept { O::iface()->_drop(O::data()); }
 
     // const methods
-    size_type shallow_size()  const  noexcept {
-        return O::iface()->shallow_size(O::data());
-    }
 
     // non-const methods (still const in router!)
     Opaque shallow_move(obj<ACollector> gc)  noexcept {