Underneath the list of modules on the left hand side of the Main navigation screen, there's a tiny double arrow and the tool tip says "Configure buttons". If you click on that and choose "Add or Remove Buttons", you can click on the "All Modules" menu choice and it will toggle off.
If you view the "Navigation Pane Options" on the same "Configure buttons", you'll see that the "All Modules" is now no longer checked, even though you can't actually uncheck "All Modules" directly like you can the rest of the buttons.
I hope this answers the question you were asking.
Best regards,
Gail J-N