postinst: remove unneeded check for deb-systemd-helper

[orndorffgrant]

Proposed Commit Message

postinst: remove unneeded check for deb-systemd-helper

deb-systemd-helper is a part of init-system-helpers, which is
Essential: yes, so the check here was superfluous.

Test Steps

Install an old version of ua that has ua-messaging.timer. Upgrade to this version and make sure ua-messaging.timer is cleaned up correctly.

Or say "this doesn't really benefit us and we don't want to bother with the extra testing here so lets just leave it as it was" - that would make sense too :) - then we can just close this PR

Desired commit type

• Squash and merge: Using the "Proposed Commit Message"
• Create a merge commit and use "Proposed Commit Message"
• Rebase and merge, no merge commit, rely only on existing commit messages

Checklist

• I have updated or added any unit tests accordingly
• I have updated or added any integration tests accordingly
• I have updated or added any documentation accordingly