chore(kinesisfirehose-alpha): cleanup the alpha module after it has entered GA (#33950)

### Reason for this change

The `kinesisfirehose` and `kinesisfirehose-destinations` modules were recently stabilized to GA in PR #33296. As such, we have no need for the alpha module any more.

### Description of changes

```sh
rm -rf aws-cdk/packages/@aws-cdk/aws-kinesisfirehose-alpha
rm -rf aws-cdk/packages/@aws-cdk/aws-kinesisfirehose-destinations-alpha
```

### Description of how you validated changes

`yarn install && npx lerna run build --skip-nx-cache` run successfully.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: Leo10Gama