slave: don't destroy session twice when user switching
    
    when user-switching from the login screen this call will succeed:
    
            migrated = try_migrate_session (slave);•
    
    Which will make this code run:
    
            if (migrated) {•
                    destroy_session (slave);•
    •
                    /* We don't stop the slave here because•
                       when Xorg exits it switches to the VT it was•
                       started from.  That interferes with fast•
                       user switching. */•
                    queue_greeter_reset (slave);•
    •
    
    queue_greeter_reset causes reset_session to get called which will
    destroy the session.  At that pint it's already destroyed because of the
    destroy_session call above.
    
    This commit drops the destroy_session call above to prevent it from getting
    destroyed twice.