remove ceilometer-expirer cron tab

We removed ceilometer expirer in queens. So this cron tab should not be configured anymore as it throws command not found errors.

In /var/spool/mail/ceilometer in undercloud we now see following error:

From <email address hidden> Tue Mar 6 00:32:46 2018
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X-Original-To: ceilometer
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Received: by undercloud-0.redhat.local (Postfix, from userid 166)
 id 574CDC957E5; Tue, 6 Mar 2018 00:32:46 -0500 (EST)
From: "(Cron Daemon)" <email address hidden>
To: <email address hidden>
Subject: Cron <ceilometer@undercloud-0> sleep $(($(od -A n -t d -N 3 /dev/urandom) % 86400)) && ceilometer-expirer
Content-Type: text/plain; charset=UTF-8
Auto-Submitted: auto-generated
Precedence: bulk
X-Cron-Env: <XDG_SESSION_ID=346>
X-Cron-Env: <XDG_RUNTIME_DIR=/run/user/166>
X-Cron-Env: <LANG=en_US.UTF-8>
X-Cron-Env: <PATH=/bin:/usr/bin:/usr/sbin SHELL=/bin/sh>
X-Cron-Env: <SHELL=/bin/sh>
X-Cron-Env: <HOME=/var/lib/ceilometer>
X-Cron-Env: <LOGNAME=ceilometer>
X-Cron-Env: <USER=ceilometer>
Message-Id: <email address hidden>
Date: Tue, 6 Mar 2018 00:32:46 -0500 (EST)

/bin/sh: ceilometer-expirer: command not found