chore(spec2cdk): remove temporary schema for AWS::AppSync::ChannelNamespace (#34459)

### Reason for this change

Schemas are not publicly available and this is no longer needed.

### Description of changes

Remove temporary schema for AWS::AppSync::ChannelNamespace

### Description of how you validated changes



### 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: paulhcsun