Changes

fileserver: remove unused fileserver.pingcap.net reference from docs-tidb-operator pipeline (#4270)

The `merged_update` pipeline for `pingcap/docs-tidb-operator` declared a
`FILE_SERVER_URL` environment variable pointing to
`fileserver.pingcap.net`, but the variable was never referenced in any
pipeline step — the pipeline only builds PDFs locally and uploads them
to Tencent Cloud via `rclone`.

## Changes

-
**`pipelines/pingcap/docs-tidb-operator/latest/merged_update.groovy`**:
Remove the dead `environment` block containing `FILE_SERVER_URL =
'http://fileserver.pingcap.net'`

```groovy
// Removed:
environment {
    FILE_SERVER_URL = 'http://fileserver.pingcap.net'
}
```

No behavioral change — the variable had no callers.

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>fileserver: migrate artifacts for
pingcap/docs-tidb-operator</issue_title>
> <issue_description>Replace fileserver.pingcap.net usage for
pingcap/docs-tidb-operator with OCI artifacts in a standard registry.
> 
> Acceptance criteria:
> - No fileserver.pingcap.net references in jobs/pipelines for
pingcap/docs-tidb-operator
> - 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#4219

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 (commit: 07498b8)