Adapt Torbutton to new session restore code in ESR 31
Since https://bugzilla.mozilla.org/show_bug.cgi?id=944557 landed our code in tbSessionStore.js is not useful anymore. The Mozilla patch is written in a way that no state is saved on disc in case PBM (which is the default) is used. I guess, we can therefore just delete that part of Torbutton.