Refine tiflash docs to remove deprecated description

[JaySon-Huang]

First-time contributors' checklist

• [x] 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 https://github.com/pingcap/tidb/issues/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).

• [x] master (the latest development version)
• [x] 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:
• 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

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[ti-chi-bot[bot]]

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In response to this:

others lgtm

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[ti-chi-bot[bot]]

@JaySon-Huang: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	f1db3acbbe6fc3e34fb1a71657f8fc742b7e5628	link	true	/test pull-verify

Full PR test history. Your PR dashboard.

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