Remove backendOK field when not used.

[ajmcmiddlin]

The backendOK field doesn't get referenced after students have pasted in new view code in Exercise 1.1. It might be worth calling this out in Exercise 1.3 when students are asked to surface the error message in loginError. It's an opportunity to clean things up, but more importantly it's an opportunity to point out that we've replaced the flag with a Maybe value.