PHPSESSID only in Backend

Is there a way to disable PHPSESSID in Kirby 2.1.1 in the Frontend? As I can see its only really used for the Backend to work and not necessarily the frontend.

When removing the session_start from /kirby/toolkit/lib/s.php the Cookie is completly removed, but then the Backend won’t work.

I’m not sure about 2.1.1 as that is pretty old, but in 2.5.12 this should not happen.

On a side note: Kirby 2 will reach EOL on December 31, 2020.