Make login not fail when user is already logged in

The MR code in https://git.drupalcode.org/project/openid_connect/-/merge_requests/41 works well. For context, this works well for our use case which initiates login from the idP (Okta, in this case) and it fixes a confusing experience where the user may have a Drupal session, but then come through the idP and get an error screen.

I applied this patch to openid_connect 3.0.0-alpha2, it works for me. Tests looks ok.

I don't think we need an option to be enable/disable the check as in the related issue, this one is good enough.

Thanks @sboden