Fix occasional start-up crash when reading GUI XMLs.

_dialog_props holds SGSharedPtrs (pointers managed by reference counters).
Explicitly casting the object to an unmanaged SGPropertyNode* and deleting it
may cause heap corruption, since the following assignment "_dialog_props[..] = ..."
also tries to delete the (already deleted) object.

src/GUI/new_gui.cxx

@@ -296,9 +296,6 @@ NewGUI::readDir (const char * path)
                 continue;
             }
             string name = nameprop->getStringValue();
-            if (_dialog_props[name])
-                delete (SGPropertyNode *)_dialog_props[name];
-
             _dialog_props[name] = props;
         }
     }