Remove the customized utility function for workflow YAML validation

[terrytangyuan]

Currently we have a utility function for validating the workflow YAML used for Argo backend in workflow_validation_utils.py. This is necessary currently since the existing release of Argo Python client v3.2.0 has issues when de-serializing an argo.workflows.client.modelsV1alpha1Template object and we have to validate specs and templates individually.

This is not ideal as the underlying schema might change as Argo gets upgraded. We should check whether the same issue exists when the new version of Argo Python client is ready and aim to reuse whatever is available.

cc @binarycrayon