dtoolbase: remove TypeHandle::_none symbol, no longer needed

dtool/src/dtoolbase/typeHandle.cxx

@@ -15,9 +15,6 @@
 #include "typeRegistryNode.h"
 #include "atomicAdjust.h"
 
-// This is initialized to zero by static initialization.
-TypeHandle TypeHandle::_none;
-
 /**
  * Returns the total allocated memory used by objects of this type, for the
  * indicated memory class.  This is only updated if track-memory-usage is set

dtool/src/dtoolbase/typeHandle.h

@@ -147,9 +147,6 @@ public:
 private:
   constexpr TypeHandle(int index);
 
-  // Only kept temporarily for ABI compatibility.
-  static TypeHandle _none;
-
   int _index;
   friend class TypeRegistry;
 };