ci: remove nightly-build.yml (startup failure from secrets-in-if; unwanted)

[adityamparikh]

Problem

nightly-build.yml fails at startup on every workflow scan — job count: 0, surfaced as a red ✗ on main pushes (e.g. run 27561117336).

Root cause is not the docker/login-action allow-list issue (this workflow has no third-party actions). It's a secrets context used in an if::

- name: Upload to Apache Nightlies
  if: ${{ secrets.APACHE_NIGHTLIES_USER != '' }}   # ← secrets not allowed in if:

The secrets context isn't available in if: conditions, so GitHub rejects the workflow at parse time ("Unrecognized named-value: 'secrets'") → startup failure, before any job runs. (Same red‑✗ symptom as the allow‑list block, different cause.)

Why remove rather than fix

The workflow's nightlies.apache.org upload step is an explicit placeholder (it only echos "Would upload to…"), and a daily nightly GitHub pre-release isn't wanted. Rather than fix the parse error to keep a half-built workflow we don't run, this removes it entirely.

Note: just commenting out the schedule: trigger would not help — the invalid file would still startup-fail and still show red on main. Deletion is the only way to fully clear it.

Changes

• Delete .github/workflows/nightly-build.yml (−216).
• dev-docs/WORKFLOWS.md: drop the nightly Quick-Reference row, the dedicated nightly-build.yml section, the Comparison-Matrix column, Scenario 3 (renumbered), and the quick-command line.
• dev-docs/DOCKER_PUBLISHING.md: drop the Nightly registry entry, the "Nightly Builds" section (renumbered), the tagging line, and the policy-note mention.

Out of scope (intentionally)

The "COMPARISON WITH OTHER WORKFLOWS" header comments in build-and-publish.yml, release-publish.yml, and atr-release.yml still list nightly-build. Left as-is here because the first two are modified by #153/#154 — touching them would create conflicts between these companion PRs.

Context

Third in a series clearing CI startup failures: #153 (build-and-publish.yml), #154 (release-publish.yml), and this one. After all three merge, the only remaining red is atr-release.yml — a stale old run on a manual "Blocked/future" workflow whose current file has no detectable startup cause.

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