ssh-server: remove deprecated variable from config (buster)

ssh-server/templates/sshd_config_stretch.j2

@@ -15,8 +15,11 @@ MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@op
 # HostKeys for protocol version 2
 HostKey /etc/ssh/ssh_host_rsa_key
 HostKey /etc/ssh/ssh_host_ed25519_key
+{# deprecated option in buster #}
+{% if ansible_distribution_major_version|int < 10 %}
 #Privilege Separation is turned on for security
 UsePrivilegeSeparation yes
+{% endif %}
 
 # Logging
 SyslogFacility AUTH