Refine tiflash docs to remove deprecated description

[JaySon-Huang]

First-time contributors' checklist

• I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added or deleted? (Required)

• The "pd buddy" has been removed from TiFlash since v6.0 ddl: Move TiFlash cluster manager module into TiDB tidb#29924

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For details, see tips for choosing the affected versions (in Chinese).

• master (the latest development version)
• v8.1 (TiDB 8.1 versions)
• v8.0 (TiDB 8.0 versions)
• v7.6 (TiDB 7.6 versions)
• v7.5 (TiDB 7.5 versions)
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• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
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Do your changes match any of the following descriptions?

• Delete files
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• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[CalvinNeo]

others lgtm

[ti-chi-bot]

@CalvinNeo: adding LGTM is restricted to approvers and reviewers in OWNERS files.

In response to this:

others lgtm

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-01-07 12:35:50.033801097 +0000 UTC m=+270693.322632806: ☑️ agreed by hfxsd.
• 2025-01-09 02:10:27.010628014 +0000 UTC m=+405970.299459709: ☑️ agreed by qiancai.

[qiancai]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #19508.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #19509.