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

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

debian/mariadb-server.postrm

@@ -12,8 +12,6 @@ fi
 
 ${DEBIAN_SCRIPT_TRACE:+ echo "#42#DEBUG# RUNNING $0 $*" 1>&2 }
 
-#DEBHELPER#
-
 #
 # - Purge logs and data only if they are ours (#307473)
 # - Remove the mysql user only after all his owned files are purged.