Remove obsolete silent pipeline from grafana

With Zuul v3 we do not have a silent pipeline, remove it from grafana
display.

Change-Id: Ie247856c08c96dcb648ddbd9f54607eaa0b5367a

grafana/zuul-status.yaml

@@ -76,15 +76,6 @@ dashboard:
               - target: stats.gauges.zuul.tenant.openstack.pipeline.release.current_changes
           type: singlestat
           valueName: current
-        - title: Silent
-          span: 2
-          sparkline:
-            full: true
-            show: true
-          targets:
-              - target: stats.gauges.zuul.tenant.openstack.pipeline.silent.current_changes
-          type: singlestat
-          valueName: current
         - title: Experimental
           span: 2
           sparkline: