IPython console: Remove on_close method from it main widget

That's not really necessary because the plugin declares the same method,
so it was being called twice when the main window is closed.

spyder/plugins/ipythonconsole/widgets/main_widget.py

@@ -350,10 +350,6 @@ def __init__(self, name=None, plugin=None, parent=None):
         # Update the list of envs at startup
         self.get_envs()
 
-    def on_close(self):
-        self.mainwindow_close = True
-        self.close_all_clients()
-
     # ---- PluginMainWidget API and settings handling
     # ------------------------------------------------------------------------
     def get_title(self):