Remove "Prioritization Note" comment from pr-governance workflow

## Summary

The `pr-governance` workflow currently posts a "📋 Prioritization Note" comment on PRs whose linked issue is not in the current sprint/milestone. The comment tags `@rajeshkamal5050` and `@kristenwomack` and asks contributors to reach out about prioritization.

I''d like to remove this check. Linked-issue enforcement should stay; the prioritization-note step should go.

## Why

- **Noise on PRs.** The note fires on every PR whose issue isn''t milestoned yet, including ones we''re already aware of and prioritizing through other channels. It adds a comment that contributors generally can''t act on.
- **Wrong escalation path.** Tagging two specific people in an automated comment creates the impression that prioritization is a 1:1 conversation, when it actually goes through milestone planning.
- **Personal mention at scale.** My handle is currently embedded in a workflow that runs on every PR — that''s not a sustainable pattern, and removing the step is cleaner than just removing the mention.

## Proposed change

Two-file change to `.github/`:

1. **`.github/workflows/pr-governance.yml`** — remove the `Check issue priority` step (the second step in `governance-checks`).
2. **`.github/scripts/pr-governance-priority-check.js`** — delete the file.

Keep:

- The `Check linked issue` step + `pr-governance-issue-check.js` (the "🔗 Linked Issue Required" comment is still useful).
- The workflow itself, its triggers, and permissions.

A side effect: the `PROJECT_READ_TOKEN` secret will become unused by this workflow. Worth checking whether anything else consumes it before considering it for cleanup (separate task).

## Acceptance criteria

- [ ] No "Prioritization Note" comment is posted on new or edited PRs.
- [ ] The "Linked Issue Required" comment continues to work.
- [ ] No CI permission or secret regressions.