Remove securityContext from manager Deployment (#751)

Removing unnecessary securityContext from manager Pod:
- runAsUser & runAsGroup are already enabled by the `USER` line of the
Dockerfile
- fsGroup is unneeded since the manager mounts no volumes.

The securityContext field overrides some of Openshift's arbitrary user
handling, while providing no benefit to the operator. Removing it
provides the same security guarantees as before, while also making
Openshift installation experience the same as any other Kubernetes
distribution.

Author: coro

config/manager/manager.yaml

@@ -30,10 +30,6 @@ spec:
         app.kubernetes.io/part-of: rabbitmq
     spec:
       serviceAccountName: rabbitmq-cluster-operator
-      securityContext:
-        runAsUser: 1000
-        runAsGroup: 1000
-        fsGroup: 1000
       containers:
       - command:
         - /manager