enh: remove ssh system sandbox check (#213)

UsePrivilegeSeparation option is deprecated.
Since the oldest supported Debian distribution is Buster (10), we can safely remove this check

Fixes #212

tests/hardening/99.5.2.8_ssh_sys_sandbox.sh

@@ -1,22 +0,0 @@
-# shellcheck shell=bash
-# run-shellcheck
-test_audit() {
-    describe Running on blank host
-    register_test retvalshouldbe 1
-    register_test contain "openssh-server is installed"
-    # shellcheck disable=2154
-    run blank "${CIS_CHECKS_DIR}/${script}.sh" --audit-all
-
-    describe Correcting situation
-    # `apply` performs a service reload after each change in the config file
-    # the service needs to be started for the reload to succeed
-    service ssh start
-    # if the audit script provides "apply" option, enable and run it
-    sed -i 's/audit/enabled/' "${CIS_CONF_DIR}/conf.d/${script}.cfg"
-    "${CIS_CHECKS_DIR}/${script}.sh" || true
-
-    describe Checking resolved state
-    register_test retvalshouldbe 0
-    register_test contain "[ OK ] ^UsePrivilegeSeparation[[:space:]]*sandbox is present in /etc/ssh/sshd_config"
-    run resolved "${CIS_CHECKS_DIR}/${script}.sh" --audit-all
-}