Remove deprecated logstash max_inflight_requests

The logstash ElasticSearch output max_inflight_requests option has been
deprecated and should not be used. Remove it.

Change-Id: I3644523f8605f17be7d9ed6f20f2020290f0fb28

modules/openstack_project/templates/logstash/indexer.conf.erb

@@ -118,6 +118,5 @@ output {
   elasticsearch {
     host => "<%= scope.lookupvar("::openstack_project::logstash_worker::discover_node") %>"
     node_name => "<%= scope.lookupvar("::hostname") %>"
-    max_inflight_requests => 512
   }
 }