br: removed the sentence about the outdated restore speed (#20973)

[ti-chi-bot]

This is an automated cherry-pick of #20973

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What is changed, added or deleted? (Required)

Correct the restore speed according to #18970

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• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
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• v5.4 (TiDB 5.4 versions)

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

• This PR is translated from:
• Other reference link(s): restore: update full restore speed #18970

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

[qiancai]

/approve

[ti-chi-bot]

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