Remove the stale galactic-router image stamp for the gateway from the publish workflow #701

Summary

The publish workflow still stamps the galactic-router image tag into the gateway's base manifests. The comment above that step says the gateway DaemonSet has two containers. It has had one since galactic-router moved to its own DaemonSet. The step does no harm only because the image string it replaces is gone from that manifest.

Acceptance criteria

• Remove the gateway base's galactic-router image stamp and its comment from the publish workflow
• Drop the matching Known Constraints entry from the gateway architecture doc

Phase 0 of #698.