Remove pre-pull Docker images step from prepare-dependencies

Docker layer cache is ephemeral per job in GitHub Actions, so images
pulled in prepare-dependencies aren't available to test jobs. Each test
job now pulls and caches its own images using Buildx with GHA cache
backend, which persists across workflow runs. The pre-pull step added
time without any benefit.

Author: zph

.github/workflows/main.yml

@@ -76,23 +76,8 @@ jobs:
         run: go mod vendor
 
       # Note: TiDB tests now use Docker images with Buildx caching instead of TiUP
-      # Docker images are cached per version in individual test jobs
-      
-      - name: Pre-pull Docker images
-        run: |
-          # Pre-pull all Docker images used in tests
-          # Note: Docker layer cache is ephemeral per job, but pre-pulling here documents
-          # what images are needed and can help with local testing
-          echo "Pre-pulling Docker images for MySQL, Percona, and MariaDB tests..."
-          docker pull mysql:5.6 || true
-          docker pull mysql:5.7 || true
-          docker pull mysql:8.0 || true
-          docker pull percona:5.7 || true
-          docker pull percona:8.0 || true
-          docker pull mariadb:10.3 || true
-          docker pull mariadb:10.8 || true
-          docker pull mariadb:10.10 || true
-          echo "Docker images pre-pull complete (images will be pulled again in test jobs with layer caching)"
+      # Docker images are cached per version in individual test jobs using Buildx with GHA cache backend
+      # Each test job pulls and caches its own images - no pre-pull needed since Docker layer cache is ephemeral per job
 
       - name: Upload Terraform binary
         uses: actions/upload-artifact@v4