Remove SealedSecret generation from the mq-spring-app chart

Based on the way the rest of the ProdRef & GitOps work has structured, we need to remove the "ownership" of the SealedSecret from the mq-spring-app chart itself and simply document a dependency on a Secret that provides the USER and PASSWORD keys that can be injected the same way.

The provenance of that Secret is outside the scope of the application, especially as it is implemented today, and would be moved to the larger GitOps/configuration role.

Tasks:
- Remove https://github.com/cloud-native-toolkit/mq-spring-app/blob/master/chart/base/values.yaml#L68-L71
- Remove https://github.com/cloud-native-toolkit/mq-spring-app/blob/master/chart/base/templates/secret.yaml
- Update values.yaml and https://github.com/cloud-native-toolkit/mq-spring-app/blob/master/chart/base/templates/deployment.yaml#L123-L132 to pull from a configurably-named Secret
- Document in README.md the dependency on the Secret (which can reference SealedSecrets as a great way to achieve the creation of the underlying Secret through GitOps) and promoting the `oc create secret generic mq-spring-app` example as the simplest dev-oriented way.
- As a follow-on, ensure that the necessary scripts & template YAMLs are created and invoked in https://github.com/osowski-demos/multi-tenancy-gitops-apps/tree/master/mq/environments/{ci,dev,staging,prod}/secrets as needed