Remove sync-asset-manager-from-master cron job from asset slave

This cron job was marked as 'absent' in this PR [1] which has been
applied in all environments and so it's now safe to remove the
definition.

[1]: #7016

modules/govuk/manifests/node/s_asset_slave.pp

@@ -41,13 +41,6 @@
     require  => File['/data/master-uploads'],
   }
 
-  cron { 'sync-asset-manager-from-master':
-    ensure  => 'absent',
-    user    => 'assets',
-    minute  => '*/10',
-    command => '/usr/bin/setlock -n /var/tmp/asset-manager.lock /usr/bin/rsync -a --delete /data/master-uploads/asset-manager/ /mnt/uploads/asset-manager',
-  }
-
   $master_metrics_hostname = regsubst($::fqdn_metrics, 'slave-\d', 'master-1')
   $graphite_mnt_uploads_metric = 'df-mnt-uploads.df_complex-used'
   $master_metric = "${master_metrics_hostname}.${graphite_mnt_uploads_metric}"