Remove allowPrivilegeEscalation from metrics-server (#8014)

"allowPrivilegeEscalation: false" blocks deploying metrics-server
on CentOS7. In addition, the original metrics-server manifest doesn't
contain it as [1]. This removes it.

[1]: 527679e5e8/manifests/base/deployment.yaml

roles/kubernetes-apps/metrics_server/templates/metrics-server-deployment.yaml.j2

@@ -67,7 +67,6 @@ spec:
           failureThreshold: 3
           initialDelaySeconds: 40
         securityContext:
-          allowPrivilegeEscalation: false
           capabilities:
             drop: ["all"]
             add: ["NET_BIND_SERVICE"]