Remove deprecated option UsePrivilegeSeparation

See https://github.com/mozilla/infosec.mozilla.org/commit/19e631ce404429e0583a98e182331cb0784f5aec

templates/sshd_config.conf.j2

@@ -45,7 +45,3 @@ LogLevel VERBOSE
 
 # Log sftp level file access (read/write/etc.) that would not be easily logged otherwise.
 Subsystem sftp  /usr/lib/openssh/sftp-server -f AUTHPRIV -l INFO
-
-# Use kernel sandbox mechanisms where possible in unprivileged processes
-# Systrace on OpenBSD, Seccomp on Linux, seatbelt on MacOSX/Darwin, rlimit elsewhere.
-UsePrivilegeSeparation sandbox