docker: remove deprecated option (#5650)

This line has been removed in respect to the OpenSSH spec update with Version 7.5 (https://www.openssh.com/txt/release-7.5)

docker/sshd_config

@@ -11,6 +11,5 @@ HostKey /data/ssh/ssh_host_ed25519_key
 PermitRootLogin no
 AuthorizedKeysFile	.ssh/authorized_keys
 PasswordAuthentication no
-UsePrivilegeSeparation no
 PermitUserEnvironment yes
 AllowUsers git