Stop trying to hide "Restore previous session"

We currently try and hide the restore session menu item, but the item is shown or hidden by mozilla code. Meaning our change does essentially nothing but change what the initial hidden attribute value is.

Moreover, I know of one other way of triggering session restore via a keyboard shortcut, which we do nothing about.

I imagine our private browsing mode makes the session restore always empty, so the default firefox logic would hopefully work for us.

So I guess the question is do we want to still try and disable this for users that manually switch on history? Note this would require a different approach than just hiding this menu item. E.g. users can select "General > Startup > Open previous windows and tabs" to do this automatically, and when this is not checked firefox gives you a notification that you can restore the session.

/cc @donuts @richard