ssh: default VerifyHostKeyDNS to no, following upstream

Revert to upstream's default. Using VerifyHostKeyDNS may depend on a
trusted nameserver and network path.

This reverts commit 83c6a5242c80160fff76fb85454938761645b0c4.

Reported by: David Leadbeater, G-Research
Reviewed by: gordon
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38648