Fully fixed the $logout() bug

@katowulf - This is my second attempt at fixing #413. My fix last time made the login event not fire after a user logs out. This is because of some hacky code we had in place to suppress multiple logout events firing. This code is not very easy to understand and is actually not necessary anymore. I added a check to logout to not fire the logout event if no user is actually logged in. This seems logical to me and I'm surprised Simple Login itself doesn't do this. This is not a breaking change since the behavior from calling `$logout()` twice was already busted. 

Note that `this._currentUserData` will get set to `null` within `_onLoginEvent()` which will get fired by the call to Simple Login's `logout()`. As a result, we no longer need to set it to `null` within `$logout()` itself.

cc/ @davideast