SSH: Remove deprecated UsePrivilegeSeparation option; rucio#311

This was deprecated in OpenSSH 7.5: https://www.openssh.com/txt/release-7.5

 * This release deprecates the sshd_config UsePrivilegeSeparation
   option, thereby making privilege separation mandatory. Privilege
   separation has been on by default for almost 15 years and
   sandboxing has been on by default for almost the last five.

ssh/sshd_config

@@ -9,8 +9,6 @@ HostKey /etc/ssh/ssh_host_rsa_key
 HostKey /etc/ssh/ssh_host_dsa_key
 HostKey /etc/ssh/ssh_host_ecdsa_key
 HostKey /etc/ssh/ssh_host_ed25519_key
-#Privilege Separation is turned on for security
-UsePrivilegeSeparation yes
 
 # Logging
 SyslogFacility AUTH