build(woodpecker): remove redundant build of a builder images

Now that the push-next job is using the forgejo action, it no longer
needs this builder image.

.woodpecker/cron-docker-builder.yml

@@ -1,15 +0,0 @@
-steps:
-
-  update-builder-image:
-    when:
-      - event: cron
-    image: docker.io/woodpeckerci/plugin-docker-buildx:latest
-    settings:
-      username: kemitix
-      repo: git.kemitix.net/kemitix/git-next-builder
-      auto_tag: true
-      dockerfile: Dockerfile.builder
-      dry-run: false # push to remote repo
-      registry: git.kemitix.net
-      password:
-        from_secret: woodpecker-docker-push