chore: remove inert legacy intiface-engine CI workflow

This workflow lived under crates/intiface_engine/.github/ from when
intiface-engine was a standalone repo. GitHub Actions only reads from
the repo root's .github/workflows/, so it was never running.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: qdot