useless secrets created for `ca` should be removed or make it useful

[bjwswang]

Well...
When I saw secrets {org}-ca and {org}-tls-ca by kubectl get ... ,I just don't know what these for....

https://github.com/hyperledger-labs/fabric-operator/blob/main/pkg/offering/base/ca/ca.go#L496

Maybe we remove these two secrets or just store the initial admin user? This only makes people confusing if it has no use. ^_^