Drop ECR multi-arch manifest step; only publish manifest to DockerHub

The DockerHub manifest is the only one consumers actually pull. The ECR
multi-arch manifest was unused, so remove the step and have the
nightly/release publish steps depend on the per-arch build steps
directly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.buildkite/release-pipeline.yaml

@@ -76,21 +76,10 @@ steps:
   - group: "Publish release images"
     key: "publish-release-images"
     steps:
-      - label: "Create multi-arch manifest"
+      - label: "Publish nightly multi-arch image to DockerHub"
         depends_on:
           - build-release-image-x86
           - build-release-image-arm64
-        id: create-multi-arch-manifest
-        agents:
-          queue: small_cpu_queue_release
-        commands:
-          - "aws ecr-public get-login-password --region us-east-1 | docker login --username AWS --password-stdin public.ecr.aws/q9t5s3a7"
-          - "docker manifest create public.ecr.aws/q9t5s3a7/vllm-omni-release-repo:$BUILDKITE_COMMIT public.ecr.aws/q9t5s3a7/vllm-omni-release-repo:$BUILDKITE_COMMIT-x86_64 public.ecr.aws/q9t5s3a7/vllm-omni-release-repo:$BUILDKITE_COMMIT-aarch64 --amend"
-          - "docker manifest push public.ecr.aws/q9t5s3a7/vllm-omni-release-repo:$BUILDKITE_COMMIT"
-
-      - label: "Publish nightly multi-arch image to DockerHub"
-        depends_on:
-          - create-multi-arch-manifest
         if: build.env("NIGHTLY") == "1"
         agents:
           queue: small_cpu_queue_release
@@ -109,7 +98,8 @@ steps:
       - block: "Publish release images to DockerHub"
         key: block-publish-release-images
         depends_on:
-          - create-multi-arch-manifest
+          - build-release-image-x86
+          - build-release-image-arm64
           - input-release-version
         if: build.env("NIGHTLY") != "1"