Removing `disk_space` from spec does not revert to default value

[mortenlj]

When a postgres service is defined with additional storage using the disk_space field on the spec, the service is updated to reflect that change.
If the disk_space field is removed from the spec in the cluster, then the operator starts reconciling, but no effect is visible on the service in the Aiven console.

To reduce disk space back to the default value, the disk_space field must be set explicitly to the default value.

Reproduce:

1. kubectl apply -f postgres-no-disk.yaml
2. Wait for service to start and become ready. Note 80GB disk.
3. kubectl apply -f postgres-add-disk.yaml
4. Wait for service to reconcile, adding disk to a total of 100GB.
5. kubectl apply -f postgres-no-disk.yaml
6. Wait ... notice that disk is still a total of 100GB, no change is made to the service.

postgres-no-disk.yaml.txt
postgres-add-disk.yaml.txt