Bug 20854: (QA follow-up) Improve readability

Bug 20854: (QA follow-up) Improve readability

The logout_cas method does too little tasks. In this case, I consider we
should not split it into a separate method. As the CAS implementation
lacked tests, it made sense to do it like this, so it is testable. But
now we have higher-level tests for logout_cas, we can bake the behavior
inside of it, and it is testable.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>