Remove the extra "nextId" variable from the "Updating arrays in state" page

[barbossa]

Hello, At the page https://beta.reactjs.org/learn/updating-arrays-in-state I've found that the "nextId" variable is not uses in the four codesandbox examples.

The first place in the "Making other changes to an array" section

The second and third place in the "Updating objects inside arrays" section

The forth place in the "Write concise update logic with Immer" section

[github-actions[bot]]

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This PR introduced no changes to the javascript bundle 🙌