Avoid redundant socket cleanup in sendto_kdc.c

Avoid redundant socket cleanup in sendto_kdc.c

Since r25120, kill_conn() has been responsible for closing and
invalidating conn->fd.  In the unlikely event that the KDC sends a TCP
response to us before we send a TCP request, we were cleaning up the
socket again, which is useless (though also harmless).  Get rid of
that code.