Remove some unused bits.

Looks like we won't need to track user logins after all, at least not
for the concept demo phase.  We can resurrect this from git history if
a need arises in the future.

Removed classes:

    FCmdrLoginMonitor
    FCmdrLogindMonitor

Removed functions:

    fcmdr_service_user_session_hold()
    fcmdr_service_user_session_release()