Dashboard provisioning: In 12.1, formerly provisioned dashboards now remain even after provisioning config is removed

### What happened? I have an automated test which passed with 12.0.x and 11.x, but is now failing with 12.1. My test is named "should be able to delete former provisioned dashboards if they are no longer provisioned". Here's how it goes: I start up a new Grafana instance using the grafana/grafana-oss docker image, and I set it up with a dashboard provisioning config that creates a single dashboard in a folder (folder created by setting `foldersFromFilesStructure: true`). The provisioning config sets `disableDeletion: false` and `allowUiEdits: true`. The test asserts that the dashboard was successfully provisioned by querying the Grafana HTTP API. Then I stop the Grafana instance and start it back up again with a different configuration: I remove the provisioning config entirely (and also the mounted directory containing the dashboard JSON itself). I then expect that the formerly provisioned dashboard is no longer present, but that its folder remains. **This fails: The formerly provisioned dashboard now remains even after the provisioning config has been removed.** The folder also remains of course, but that was expected. ### What did you expect to happen? I expected my above test to pass when I upgraded to 12.1, as it had for 12.0.x and 11.x versions of Grafana. ### Did this work before? Yes. It worked in Grafana 12.0.x and 11.x. ### How do we reproduce it? 1. Prepare a `dashboards.yml` that permits deletion of dashboards: The one from my test: ``` apiVersion: 1 providers: - name: 'gdev dashboards' folder: '' folderUid: '' type: file allowUiUpdates: true options: path: /provisioned-dashboards foldersFromFilesStructure: true ``` 2. Prepare a test dashboard JSON: The one from my test: ``` {"title":"Global Volume","uid":"global-volume","drv":{"version":"1.2.10","type":"built_in"}} ``` 3. Put the dashboard JSON inside a local directory to serve as the Grafana folder. In my test, the directory (and therefore the Grafa…