cmDefinitions::Def: remove custom copy ctor

Source/cmDefinitions.h

@@ -65,12 +65,6 @@ private:
       , Used(false)
     {
     }
-    Def(Def const& d)
-      : std_string(d)
-      , Exists(d.Exists)
-      , Used(d.Used)
-    {
-    }
     bool Exists;
     bool Used;
   };