Fix segfault when adding more tabs to Database editor

Removed a slot decoration from method which, for unknown reason, seems
to fix two segfaults:

- Segfault when adding new tabs to the editor using the '+' button
- Segfault when switching between tabbed Object/Relationship
  parameter value and default value docs in a freshly created tab

Re #2262

spinetoolbox/spine_db_editor/widgets/spine_db_editor.py

@@ -986,7 +986,6 @@ def _restart_timer_refresh_tab_order(self, _visible=False):
         self._timer_refresh_tab_order.timeout.connect(self._refresh_tab_order, Qt.UniqueConnection)
         self._timer_refresh_tab_order.start(100)
 
-    @Slot()
     def _refresh_tab_order(self):
         if self._torn_down:
             return