MDEV-34146: Remove duplicate #DEBHELPER# from MariaDB server postrm

[illuusio]

• The Jira issue number for this PR is: MDEV-34146

Description

Moving to use Debian systemd install and uninstall scripts caused duplicated DEBHELP to server postrm script. Commit removes unneeded and makes postrm work better and pass lintian tests

Release Notes

No notes are needed

How can this PR be tested?

Build DEB packages with debian/deb-autobake.sh and run lintian --display-info --pedantic --allow-root ../*.changes to check if E: mariadb-server: duplicate-updaterc.d-calls-in-postrm mariadb [postrm:86] is not there PR works

or see Salsa-CI run's lintian:
https://salsa.debian.org/illuusio/mariadb-server/-/commit/ce8ff3539d17a661ec67a347b4180abe808c11ff

Basing the PR against the correct MariaDB version

• This is a bug fix and the PR is based against the earliest maintained branch in which the bug can be reproduced.

PR quality check

• I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.