Revert "add multi value index case from cnf and dnf combination"

[ran-huang]

Reverts pingcap/docs#15923

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PR needs rebase.

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[qiancai]

Removed the needs-cherry-pick-release-7.6 label because the v7.6 docs have been archived at https://docs-archive.pingcap.com/tidb/v7.6 and will no longer receive new updates.