puppet: Disable log2zulip cron job.

It hasn't been working for years, but more importantly, it spams up
root's mail queue so that one can't find important things in there
(e.g. the fact that the long-term-idle cron job was failing).

puppet/zulip_ops/manifests/app_frontend.pp

@@ -27,11 +27,7 @@ class zulip_ops::app_frontend {
   }
 
   file { '/etc/cron.d/log2zulip':
-    ensure => file,
-    owner  => 'root',
-    group  => 'root',
-    mode   => '0644',
-    source => 'puppet:///modules/zulip_ops/cron.d/log2zulip',
+    ensure => absent,
   }
 
   file { '/etc/cron.d/check_send_receive_time':