T4856: Fix IPsec DHCP-client exit hook

The script acually does not have the variable secrets_lines and
secret lines itself does not have the marker # dhcp:{interface}
in to_find

Needs to rewrite this script in the future if it is required

This commit fixes DHCP-client exit hook:

dhclient[6800]: NameError: name 'secrets_lines' is not defined
root[6801]: /etc/dhcp/dhclient-exit-hooks.d/99-ipsec-dhclient-hook returned non-zero exit status 1

(cherry picked from commit a9cf7246d4450c8b3e1b749b36c3393b0963404b)