Revert previous: Don't generate XMSS host keys for sshd by default.

XMSS is a stateful post-quantum signature scheme.

- Post-quantum security for _online_ authentication is not important
  until quantum computers become practical; there's no danger of
  retroactive forgery in sessions that have already completed.

- As a stateful signature schemes, XMSS is qualitatively different
  from all the other ones sshd supports, requiring additional
  administrative care: roll back the state (e.g., from a disk backup
  or VM snapshot), and you've shot yourself in the foot.

If users want XMSS keys, they can make them explicitly, but there's
no need for this to be enabled by default.

Discussed with christos offline.

etc/rc.d/sshd

@@ -1,6 +1,6 @@
 #!/bin/sh
 #
-# $NetBSD: sshd,v 1.28 2018/05/26 00:17:54 jmcneill Exp $
+# $NetBSD: sshd,v 1.29 2018/05/26 19:18:11 riastradh Exp $
 #
 
 # PROVIDE: sshd
@@ -37,7 +37,6 @@ dsa	1024	ssh_host_dsa_key	2	DSA
 ecdsa	521	ssh_host_ecdsa_key	1	ECDSA
 ed25519	-1	ssh_host_ed25519_key	1	ED25519
 rsa	0	ssh_host_rsa_key	2	RSA
-xmss	0	ssh_host_xmss_key	1	XMSS
 _EOF
 )
 }