This PR removes an unused code path in _generate_yaml()in sceptre/cli/helpers.py

yaml.safe_loads() does not exist, so that code path has never done anything.

PR Checklist

• [✓ ] Wrote a good commit message & description [see guide below].
• [ ✓] Commit message starts with [Resolve #issue-number].
• [ ✓] Added/Updated unit tests.
• Added/Updated integration tests (if applicable).
• [ ✓] All unit tests (poetry run tox) are passing.
• [ ✓] Used the same coding conventions as the rest of the project.
• [ ✓] The new code passes pre-commit validations (poetry run pre-commit run --all-files).
• [ ✓] The PR relates to only one subject with a clear title.
  and description in grammatically correct, complete sentences.

Approver/Reviewer Checklist

• Before merge squash related commits.

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