SecurityLevelPreferences.uninit() never called on exit of about:preferences

Found this while building out a skeleton for #31286 (moved). Looks like I didn't actually register the unload event callback, so the object's observer callback is never removed. Worst case scenario the 'observe' callback will be called for every time the user has entered about:preferences and repeatedly edit the XUL to reflect the changed security_slider value. Not really a problem but I'll have a fixup patch shortly.