puppet: Remove backups cron job if they are no longer enabled.

puppet/zulip/manifests/postgresql_base.pp

@@ -104,5 +104,9 @@ class zulip::postgresql_base {
   $backups_directory = zulipconf('postgresql', 'backups_directory', '')
   if $backups_s3_bucket != '' or $backups_directory != '' {
     include zulip::postgresql_backups
+  } else {
+    file { '/etc/cron.d/pg_backup_and_purge':
+      ensure  => absent,
+    }
   }
 }