Microsoft.AppConfiguration: 2020-07-01-preview can't delete keyValue

```json
{
"statusCode": "MethodNotAllowed",
"serviceRequestId": "09cbb962-d631-408a-9df0-97c83f040500",
"statusMessage": "{\"error\":{\"code\":\"BadRequest\",\"message\":\"\"}}",
"eventCategory": "Administrative",
"entity": "/subscriptions/393dab78-686f-41be-b9d5-f244fb11eba7/resourceGroups/test-rg/providers/Microsoft.AppConfiguration/configurationStores/cfg/keyValues/cityu",
"message": "Microsoft.AppConfiguration/configurationStores/keyValues/delete",
"hierarchy": "9b62c0fc-ad42-4789-b7e9-0d9e03e1e11f"
}
```

Can't delete KeyValue resource using ARM template with version 2020-07-01-preview @ZhijunZhao @ jimmyca15

Start by reproducing deletion of the Microsoft.AppConfiguration keyValue through an ARM template with API version 2020-07-01-preview. Then locate the corresponding REST API specification and compare the delete operation with the reported MethodNotAllowed response. Done means the keyValue can be deleted successfully without that response.

Written by the indexing model from the issue text.