Refine tiflash docs to remove deprecated description

[qiancai]

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

Which TiDB version(s) do your changes apply to? (Required)

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

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)
• v7.4 (TiDB 7.4 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)

What is the related PR or file link(s)?

• This PR is translated from: Refine tiflash docs to remove deprecated description docs-cn#17017
• Other reference link(s):

Do your changes match any of the following descriptions?

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

[JaySon-Huang]

LGTM

Thank a lot!

[ti-chi-bot]

@JaySon-Huang: adding LGTM is restricted to approvers and reviewers in OWNERS files.

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LGTM

Thank a lot!

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-01-07 12:35:48.09534953 +0000 UTC m=+270691.384181235: ☑️ agreed by hfxsd.

[qiancai]

/approve

[qiancai]

/approve

[ti-chi-bot]

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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-8.1: #19946.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #19947.