Remove deprecated approverpolicy_certificaterequest_ metrics

In https://github.com/cert-manager/approver-policy/pull/712 we deprecated the following metrics:

- `approverpolicy_certificaterequest_approved_count`
- `approverpolicy_certificaterequest_denied_count`
- `approverpolicy_certificaterequest_unmatched_count`

These metrics should be removed after a couple of releases.