Changes

pipelines(docs-cn): remove fileserver.pingcap.net references (#4269)

Both `pingcap/docs-cn` pipelines carried a `FILE_SERVER_URL =
'http://fileserver.pingcap.net'` environment variable that was never
referenced in any pipeline step — dead config from a prior iteration.

## Changes

- **`pipelines/pingcap/docs-cn/latest/pull_verify.groovy`** — drop
unused `environment` block containing `FILE_SERVER_URL`
- **`pipelines/pingcap/docs-cn/latest/merged_update_docs_cn.groovy`** —
same

No artifact migration to OCI is required: neither pipeline fetches
artifacts from fileserver. The `merged_update_docs_cn` pipeline builds
its PDF in-container and uploads directly to Tencent Cloud;
`pull_verify` downloads check scripts from `raw.githubusercontent.com`.

<!-- START COPILOT ORIGINAL PROMPT -->



<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>fileserver: migrate artifacts for
pingcap/docs-cn</issue_title>
> <issue_description>Replace fileserver.pingcap.net usage for
pingcap/docs-cn with OCI artifacts in a standard registry.
> 
> Acceptance criteria:
> - No fileserver.pingcap.net references in jobs/pipelines for
pingcap/docs-cn
> - Artifacts pulled via OCI registry with documented paths
> </issue_description>
> 
> <agent_instructions>Please implement this using prow-jobs when
possible.</agent_instructions>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>



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- Fixes PingCAP-QE/ci#4220

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Co-authored-by: wuhuizuo <2574558+wuhuizuo@users.noreply.github.com>
Co-authored-by: wuhuizuo <wuhuizuo@126.com>
 (commit: f603eb1)