Remove host-credentials flag and host field of PostgreSQLDatabase

[Crevil]

With the implementation of PostgreSQLHostCredentails CR (#90) the existing host-credentials flag and the host field of the PostgreSQLDatabase resource becomes redundant.

I think we should remove these and only rely on the new resource to avoid the complexity in the controller and ensuring having a "single way" of setting the controller up.

[Crevil]

We will have to do a migration for all resources before this can be rolled out so I think we should have a release with the new credentials setup after which we can then remove the flag and cut another release. That way it is possible to upgrade in two steps.