[BUG] Incomplete migration of Microsoft.Insights to typespec

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/main/specification/monitor/resource-manager/Microsoft.Insights/Insights/preview/2021-05-01-preview/openapi.json

### API Spec version

2021-05-01-preview

### Describe the bug

While the spec has been migrated to typespec, it seems the original spec has been left in place.

New spec:
[specification/monitor/resource-manager/Microsoft.Insights/Insights/preview/2021-05-01-preview/openapi.json](https://github.com/Azure/azure-rest-api-specs/blob/main/specification/monitor/resource-manager/Microsoft.Insights/Insights/preview/2021-05-01-preview/openapi.json)

Old spec:
[specification/monitor/resource-manager/Microsoft.Insights/Insights/preview/2021-05-01-preview/diagnosticsSettings_API.json](https://github.com/Azure/azure-rest-api-specs/blob/main/specification/monitor/resource-manager/Microsoft.Insights/Insights/preview/2021-05-01-preview/diagnosticsSettings_API.json)

Both versions appear to define the same API - `DiagnosticsSettings` is one example of a duplicate, though they have subtly different shapes.

**Why has the old version not been removed, as most other migrated resources have done?**

Having both present is tripping up the [Azure Service Operator](https://github.com/Azure/azure-service-operator) code generator, which is how I discovered this.

### Expected behavior

Once the typespec migration is complete, the old file are usually deleted to avoid the problem of duplicate definitions. That hasn't happened here.

### Actual behavior

The old spec is still present.

### Reproduction Steps

-

### Environment

_No response_