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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• v8.1 (TiDB 8.1 versions)
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• 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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• Might cause conflicts after applied to another branch

[CalvinNeo]

others lgtm

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