prometheus: upgrade prometheus to 2.3.1

- remove unused variables

roles/openshift_prometheus/defaults/main.yaml

@@ -8,7 +8,7 @@ openshift_prometheus_namespace: openshift-metrics
 # Need to standardise these tags
 l_openshift_prometheus_version_dict:
   origin:
-    prometheus: 'v2.2.1'
+    prometheus: 'v2.3.1'
     alert_manager: 'v0.15.0'
     alert_buffer: 'v0.0.2'
     node_exporter: 'v0.16.0'
@@ -41,9 +41,6 @@ openshift_prometheus_hostname: prometheus-{{openshift_prometheus_namespace}}.{{o
 openshift_prometheus_alerts_hostname: alerts-{{openshift_prometheus_namespace}}.{{openshift_master_default_subdomain}}
 openshift_prometheus_alertmanager_hostname: alertmanager-{{openshift_prometheus_namespace}}.{{openshift_master_default_subdomain}}
 
-l_openshift_prometheus_image_version: "{{ openshift_prometheus_image_version | default('v2.2.1') }}"
-l_openshift_prometheus_alertbuffer_image_version: "{{ openshift_prometheus_alertbuffer_image_version | default('v0.0.2') }}"
-
 openshift_prometheus_node_selector: "{{ openshift_hosted_infra_selector | default('node-role.kubernetes.io/infra=true') | map_from_pairs }}"
 
 openshift_prometheus_service_port: 443