Remove optional ref: master from setup-frontend action (#5858)

## Summary
- Removed the optional `ref: master` parameter from the ComfyUI checkout
step in the setup-frontend action
- The ref parameter defaults to the repository's default branch when
omitted

## Details
The `ref: master` specification in
`.github/actions/setup-frontend/action.yml` is unnecessary since GitHub
Actions will automatically use the repository's default branch when the
ref parameter is not provided.

This simplifies the configuration and makes it more maintainable, as the
action will automatically follow any future changes to the default
branch name.

## Test plan
- [ ] Verify that GitHub Actions workflows using this composite action
still work correctly
- [ ] Confirm ComfyUI is checked out properly in CI/CD pipelines

Generated with [Claude Code](https://claude.ai/code)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-5858-Remove-optional-ref-master-from-setup-frontend-action-27e6d73d365081aeb632f2d0e76f267d)
by [Unito](https://www.unito.io)

Co-authored-by: Claude <[email protected]>

Author: snomiao

.github/actions/setup-frontend/action.yml

@@ -13,7 +13,6 @@ runs:
       with:
         repository: 'comfyanonymous/ComfyUI'
         path: 'ComfyUI'
-        ref: 'master'
 
     - name: Checkout ComfyUI_frontend
       uses: actions/checkout@v4