workflow: drop otk validate as it is now redundant

This step is redundant now that we have
`test_against_images_refs.py` as this runs `otk.compile()
internally.

.github/workflows/test.yml

@@ -28,8 +28,3 @@ jobs:
           pip install .[dev]
       - name: Run unit tests
         run: PYTHONPATH=./src make test
-      - name: Validate OTK configuration
-        run: |
-          for f in example/*/*.yaml; do
-            otk -v validate "$f"
-          done