[-] FO : Fix multiple "Add to cart" event when block layered is used

This issue was first discovered on IE, but we also noticed that the event is also fired on Firefox/Chrome but does fire add to cart process.
By adding off() to theses events, we prevent this behaviour to happen when using ajaxCart.overrideButtonsInThePage() method multiple time on a same page.
This is a simple inherit from the previous ajaxCart version (PS 1.5) that is using unbind() and already preventing this behaviour.