[ECS] [BUG]: ECS service update of `serviceRegistries` fails since recent changes

Steps to reproduce:

* Create a CFN template with task/service definitions
* Add a Service discovery resource, `AWS::ServiceDiscovery::Service`
* Use the service discovery resource and use it in `serviceRegistries` of the ECS Service (`Registries` in CFN).
* Create stack
* Update template to remove the `AWS::ServiceDiscovery::Service` resource
* Set the value of `Registries` (`serviceRegistries` via API) to `[]` (empty list).
* Update stack.

CFN will delete the `AWS::ServiceDiscovery::Service` resource but ECS still persists it in the configured registries.

Also this cannot be changed in the console?

Start by reproducing the issue with the listed CloudFormation template, AWS::ServiceDiscovery::Service, ECS service, and empty serviceRegistries update. Check whether the ECS service and console paths expose the registry removal behavior; done means removing the resource and setting Registries to [] no longer leaves the registry configured.

Written by the indexing model from the issue text.