553 - Remove unnecessary screens from NUX sign in

Issue: https://github.com/wordpress-mobile/WordPress-iOS/issues/553

New launch screen:
![ios simulator screen shot nov 17 2013 2 56 26 pm](https://f.cloud.github.com/assets/3904502/1559611/80030198-4fdb-11e3-9802-ce229c839358.png)