ci: remove vestigial in-cluster build pipeline (ADR-0002 decommission, infra#31)

hmrc-sync has no live consumer of an in-cluster-built image; build pipeline retired.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.woodpecker.yml

@@ -1,28 +0,0 @@
-when:
-  event: push
-
-clone:
-  git:
-    image: woodpeckerci/plugin-git
-    settings:
-      attempts: 5
-      backoff: 10s
-
-steps:
-  - name: build-and-push
-    image: woodpeckerci/plugin-docker-buildx
-    settings:
-      # Phase 4 of forgejo-registry-consolidation 2026-05-07 — Forgejo only.
-      repo:
-        - forgejo.viktorbarzin.me/viktor/hmrc-sync
-      logins:
-        - registry: forgejo.viktorbarzin.me
-          username:
-            from_secret: forgejo_user
-          password:
-            from_secret: forgejo_push_token
-      dockerfile: Dockerfile
-      context: .
-      auto_tag: true
-      platforms:
-        - linux/amd64