Remove placeholder CI workflow files

Delete notes-api-ci-validation-only.yml and notes-api-ci-workflow.yml
as they were placeholder files that were unintentionally triggering on
every push to main/dev branches.

Only ci.yml should be active in the CI pipeline.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: christosm-dev