Remove insecure ciphers from GCE sshd configuration

They were added for unclear reasons in r277263. The current OpenSSH
defaults (7.5+) are reasonable, and do not include the insecure rc4 cipher:

chacha20-poly1305@openssh.com,
aes128-ctr,aes192-ctr,aes256-ctr,
aes128-gcm@openssh.com,aes256-gcm@openssh.com,
aes128-cbc,aes192-cbc,aes256-cbc

I think I recall there being a reason for a specific list of ciphers on GCE
at the time, but I do not recall what it was, and cannot find any
current GCE documentation of such a list.

So, just revert the explicit configuration and use sane openssh defaults.

PR: 230092
Submitted by: Gustavo Scalet <gustavo.scalet AT collabora.com>
MFC after: 3 days
Security: yes