Remove explicit kill of privsep preauth child's PID in SIGALRM handler.

Remove explicit kill of privsep preauth child's PID in SIGALRM handler.
It's no longer needed since the child will get terminated by the SIGTERM
to the process group that cleans up any auth helpers, it simplifies
the signal handler and removes the risk of a race when updating the PID.
Based on analysis by HerrSpace in github PR#289, ok djm@