Removing `readOnly: true` on response-only property is flagged as error

While migrating Key Vault specs from Swagger to TSP, we [removed](https://github.com/Azure/azure-rest-api-specs/pull/28708/commits/bde57be10ffbd04acc86fe2d441e4aae8a62bbbc) a `readOnly` designation for a property on a response-only type. This was [flagged as an error](https://dev.azure.com/azure-sdk/internal/_build/results?buildId=4481832&view=logs&j=98f87804-8e1f-5655-af02-e80aefa7aa97&t=65457206-7ab6-5966-2a2f-9ae3efa14a03&l=931-955) by the OpenAPI diff; per context in [this review thread](https://github.com/Azure/azure-rest-api-specs/pull/28708#discussion_r1917533739), this error doesn't seem to be correct.