Remove Exit Kiosk Mode button

[danielfe]

In a future, possibly in a native app, when kiosk mode locks the user in to that full screen experience we'll need a way to exit it. But at present Kiosk mode opens in a separate tab and the user can return to HCH just by opening a new tab and putting in the HCH URL.

In the demo of the latest kiosk mode we had an Exit Kiosk mode link. For now let's just remove that link and let the user close the window if they want to.