nanobsd: remove unmodified copies of ssh config files

Nanobsd included copies of ssh_config and sshd_config. The former is
identical to the one provided by the base system, and the latter is
identical except for PermitRootLogin, which is updated by nanobsd's
cust_allow_ssh_root anyhow. Remove nanobsd's copies and use the
existing base system ones.

Reported by: Jose Luis Duran <jlduran@gmail.com> in D34937
Reviewed by: Jose Luis Duran <jlduran@gmail.com>, imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36933