clean(semantic.yml): deleted Semantic Pull Request config (#19964)

## Summary

This PR removes the config file for Semantic Pull Requests. This should no longer be needed since we've replaced it with a custom action [`tools/@aws-cdk-prlint`](https://github.com/aws/aws-cdk/blob/master/tools/%40aws-cdk/prlint/lint.ts#L31-L41)

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### All Submissions:

* [x] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/master/CONTRIBUTING.md)

### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/master/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/master/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

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

Author: ryparker