if you want to remove the auto login functionality, you will just need to delete the second Authenticate user behavior that is applied to the login page.
Look in the server behaviors panel for the login page, you will see that there are t 2 security assist Authenticate User Behaviors, Double click each one and look at the triggers. One is using a form submit trigger, the other is using the cookies as a trigger.
Delete the one that uses the cookies as the trigger to remove the auto login function.