Disconnecting a branch does not clean up notifications for it

### Before opening, please confirm:

- [X] I have checked to see if my question is addressed in the [FAQ](https://github.com/aws-amplify/amplify-console/blob/master/FAQ.md).
- [X] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-console/issues?q=is%3Aissue+).
- [X] I have read the guide for [submitting bug reports](https://github.com/aws-amplify/amplify-console/blob/master/CONTRIBUTING.md).
- [X] I have done my best to include a minimal, self-contained set of instructions for consistently reproducing the issue.

### App Id

d2ibhgok7yafet

### Region

us-east-1

### Amplify Console feature

Notifications

### Describe the bug

Adding 2 new connected branches and deleting 2 old ones left us in an inconsistent state:
1. CloudWatch event rules exist for all 4 branches
2. Amplify console shows 2 old branches under "Notifications". In the edit dialog branch name appears as blank, and if I choose a new branch name, saving has no effect.

### Expected behavior

1. Deleting a connected branch in AWS Amplify console should delete a corresponding CloudWatch Events Rule.
2. AWS Amplify console should not display deleted branches anywhere.

### Reproduction steps

1. Connect a new branch (`A`), configure notifications for it
2. Connect a new branch (`B`), delete the first one (`A)`
3. Attempt to configure notifications for branch `B`

### Build Settings

_No response_

### Additional information

Connected branches in Amplify console:

Notifications in Amplify Console:

View in CloudWatch console - the rules marked in orange should have been deleted:

Workaround:
1. Add new notification rules for the new branches using the same email. Confirm subscription.
2. Delete existing notification rules related to now-deleted branches.

This makes Amplify show correct branch names:

However, CloudWatch Event Rules are still present for now-deleted branches.

Reproduce the branch lifecycle in the Notifications feature: connect branches A and B, configure notifications for A, delete A, and configure notifications for B. Inspect the resulting Amplify Console notification entries and CloudWatch Event Rules; done means deleted branches no longer appear and their corresponding rules are removed.

Written by the indexing model from the issue text.