perf(components): [table] remove redundant updateSelectionByRowKey

[OFreshman]

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This PR builds upon #20995, which addressed the issue where child nodes in a tree structure could not be selected after toggling pages when reserve-selection was enabled.

Since the issue has been resolved by copying the data properly, the updateSelectionByRowKey function is now redundant. To improve performance and reduce unnecessary computation, this function has been removed.

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npm i https://pkg.pr.new/element-plus/element-plus@21092

commit: f59f22a

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🧪 Playground Preview: https://element-plus.run/?pr=21092
Please comment the example via this playground if needed.

[warmthsea]

LGTM

[OFreshman]

Hi, kindly pinging for a review when convenient 🙇
Let me know if anything needs to be adjusted.

[Dsaquel]

Good catch!