Remove unused variables (#6574)

Author: smith558

src/content/learn/adding-interactivity.md

@@ -646,7 +646,6 @@ Arrays are another type of mutable JavaScript objects you can store in state and
 ```js
 import { useState } from 'react';
 
-let nextId = 3;
 const initialList = [
   { id: 0, title: 'Big Bellies', seen: false },
   { id: 1, title: 'Lunar Landscape', seen: false },
@@ -714,7 +713,6 @@ If copying arrays in code gets tedious, you can use a library like [Immer](https
 import { useState } from 'react';
 import { useImmer } from 'use-immer';
 
-let nextId = 3;
 const initialList = [
   { id: 0, title: 'Big Bellies', seen: false },
   { id: 1, title: 'Lunar Landscape', seen: false },