Don't use Old AutoLogin when Silent is used

When CentralAuth's Selent mode is used, $wgCentralAuthUseOldAutoLogin
is not intended to be used except in weird, mixed environments.

Now that all users are on SUL2, disable $wgCentralAuthUseOldAutoLogin
when $wgCentralAuthSilentLogin is used.

Change-Id: I8d47970ad74c746b5cf5b4da83faa704efacdc97