Heads-up: removing outdated OpenSSH ciphers

[Ed Maste]

A base system OpenSSH update in 2016 or so removed a number of ciphers
from the default lists offered by the server/client (due to known
weaknesses). This prompted FreeBSD PR207679, and they were restored in
r296634.

It's time to retire this local change against upstream OpenSSH; the
ciphers should still be available via a configuration option if anyone
needs them but they should not be offered by default.

Review D25833 is open for the change: https://reviews.freebsd.org/D25833