[lib] Remove unnecessary code

Summary: Since D9500 sessionChange is obligatory in AuthErrorServerSocketMessage. This was dead code.

Test Plan: flow. I checked all places where AUTH_ERROR is returned by the keyserver. They do indeed all specify sessionChange.

Reviewers: michal

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10289