loginDialog: fix cancel button in ask for username mode

If the user clicks Not Listed? to enter ask for username mode, clicks
cancel, and then attempts to log in via the user list, the user will see
"Authentication failed" after correctly typing the password, and then
will become stuck in an empty screen with just the gray noise background.

The problem is, we forgot to disconnect from the signal that's waiting
for the next button to be pressed on the username entry screen. Since
the signal handler that executes here is expecting the username to be
input, and isn't prepared for us to have switched back to user list,
various bad things happen. We try to start two gdm-password
conversations at once, for instance, one using the user's password as
the username. I stopped investigating here, because it's easy to fix by
disconnecting from the signal at the right time.

https://bugzilla.gnome.org/show_bug.cgi?id=770328