[ECR] [request]: Have the service retire KMS grants on repo deletion

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**Tell us about your request**
ECR now supports KMS encryption. Using it causes an error on deletion, where the service doesn't retire the KMS grant and tries to use the role to do so, which isn't congruent with other services and requires unreasonable permissions _or_ manual cleanup.

**Which service(s) is this request for?**
ECR

No implementation files, tests, or entry points are identified. Start by reviewing how ECR handles KMS-encrypted repositories during deletion and compare its grant-retirement behavior with other services; done means repository deletion retires the KMS grant without requiring unreasonable permissions or manual cleanup.

Written by the indexing model from the issue text.