oidc configuration not being removed when config is unset

[James Simpson]

I have the following config options set via Juju:
 * oidc_client_id
 * oidc_client_secret
 * oidc_issuer
 * oidc_logout_url

These are correctly reflected in "/etc/landscape/service.conf":

```
ubuntu@juju-979dd6-stg-is-landscape-testing-88:~$ sudo grep oidc /etc/landscape/service.conf
oidc-issuer = https://accounts.google.com
oidc-client-id = <redacted>
oidc-client-secret = <redacted>
oidc-logout-url =
```

When I set those config options back to their defaults:

```
juju config landscape-server --reset oidc_client_id --reset oidc_client_secret --reset oidc_issuer --reset oidc_logout_url
```

I see the following in the unit log:

```
2024-03-27 03:58:16 INFO unit.landscape-server/22.juju-log server.go:325 Starting services
2024-03-27 03:58:35 INFO juju.worker.uniter.operation runhook.go:186 ran "config-changed" hook (via hook dispatching script: dispatch)
```

But the OIDC keys are not removed from "/etc/landscape/service.conf".

As a workaround I can set "additional_service_config" as below:

```
stg-is-landscape-testing@is-bastion-ps6:~/plan$ juju config landscape-server additional_service_config

[landscape]
oidc-issuer =
oidc-client-id =
oidc-client-secret =
oidc-logout-url =
```

This blanks out those settings:
```
ubuntu@juju-979dd6-stg-is-landscape-testing-88:~$ sudo grep oidc /etc/landscape/service.conf
oidc-issuer =
oidc-client-id =
oidc-client-secret =
oidc-logout-url =
```