didSkipOptionalLogin flag is not cleaned up properly

The `kBITAuthenticatorDidSkipOptionalLogin` key was not being reset upon cleanup because it is stored in `NSUserDefaults` rather than the keychain. The cleanup method was removing it from the keychain.

This probably does not affect normal use, but it was causing issues with an approach we are taking to avoid showing the authenticator until the user enables a "private updates" setting in the app. After skipping the authenticator, it would never show up again until the app was uninstalled because the skip flag was never reset. I'll add a separate feature request so that we can delay authentication in a less hackish way.