[ECS] Untag removed tags from ECS service

[t-kikuc]

What would you like to be added:

When a user removes a tag from serviceDefinition, then that tag should be removed on AWS.

Why is this needed:

Now we use TagResource() but don't use UntagResource() for ECS, so the removed tags are not removed from ECS service.

[t-kikuc]

My idea:

1. UpdateService() should include tag/untag to the service, and the response should contain the tags.

   • That's because the response should be the same as the real status in AWS including tags.

   • We might need to change the func name likeReconcileService()

2. The below line should be refactored(removed) along with (1) https://github.com/pipe-cd/pipecd/blob/0d81adf1ebc7eef3c8c7a630b0c201ce4afc1c24/pkg/app/piped/executor/ecs/ecs.go#L159