Remove DNSSEC config management in bootstrap-debian.yml (#7408)

DNSSEC is off by default on ubuntu/bionic64 (18.04) as per resolved.conf(5).
These tasks are artefacts of obsolete infra configuration, and no longer needed.

Further removing these tasks resolves the issue that the tasks always reports
'changed' and bounces systemd-resolved unneccesarily, even if there was no
actual modification of /etc/systemd/resolved.conf.