Add removeAltInterceptor method

In case a ViewController is detached after a new ViewController was
already attached, we have to check whether the altAuthInterceptor is the
correct one to remove, else we could unintentionally set
altAuthInterceptor=null when it shouldn't be null.

Test: manual
Fixes: 189328154
Change-Id: If04ca36b1259083e552db8fd33706f8d176ce949