[logstash] use only httpPort in headless service

This commit fix an issue introduced in 8ed75a9 where helm
install/upgrade is failing if a NodePort service is defined (see [0] for
more details).

Logstash headless service used to manage Statefulset does not anymore include
`.Values.service.ports` and include only Logstash httpPort.

This is done because headless service `ports` doesn't accept `nodePort`
values that can be included in `.Values.service.ports` when adding a
NodePort service.

[0]: elastic#807

logstash/templates/service-headless.yaml

@@ -16,9 +16,5 @@ spec:
   selector:
     app: "{{ template "logstash.fullname" . }}"
   ports:
-{{- if .Values.service }}
-{{ toYaml .Values.service.ports | indent 4 }}
-{{- else }}
     - name: http
       port: {{ .Values.httpPort }}
-{{- end }}