Hide logout button from native FxA (bug 1073177)

https://bugzilla.mozilla.org/show_bug.cgi?id=1073177

Hide the logout button from fireplace for native Firefox Accounts users.


![no-logout](https://cloud.githubusercontent.com/assets/211578/4474447/fb810cb0-495f-11e4-95d4-652057aca833.png)