Cleaner way to check for successful login

Snapchat will return ``logged: true`` upon successful login, so just
use that. Setting ``Snapchat::$username`` using the username returned
from the API call is pointless as this should be the same.
Revert "Cleaner way to check for successful login"

This reverts commit adca52ea14947bfa4e5b9bbf26f3d459903dd185.
Cleaner way to check for successful login

Snapchat will return ‘logged: true’ upon successful login, so use that
instead (more fail-safe). Snapchat will sometimes return ‘username:
false’ so don’t set Snapchat::$username to the value gotten from the
API.