Prometheus.service.annotations in the default values.yaml should be unbiased

Right now default values.yaml set biased annotation on coredns prometheus metrics service object.

```yaml
prometheus:
service:
enabled: false
annotations:
prometheus.io/scrape: "true"
prometheus.io/port: "9153"
```

- default values in the most cases should be blank literals
- current annotations setup is biased, putting this very specific annotations

What if someone does not want to use any scraping at all?
What if someone is not using this native prometheus "hack" and is using prometheus operator workflow with ServiceMonitors?

In both above cases I have to override default value for these annotations with `null` to get completely rid of them.
```yaml
prometheus:
service:
enabled: true
annotations: null
monitor:
enabled: true
```

Yep, these annotations won't hurt in most cases, and can be just ignored. But we are returning to the default case. Default values in the chart's values.yaml should be blank.

If a person wants to scrape coredns metrics using these annotations, this person has to explicitly enable annotations for his specific setup. That also makes setup more efficient, explicit, and trackable in GitOps way.