chore(lint): remove unused var (#11236)

### Description

Remove unused var

### Changelog

**Removed**

- unused var

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packages/carbon-web-components/src/components/checkbox/checkbox-story.ts

@@ -10,7 +10,6 @@
 import { html } from 'lit';
 import { action } from '@storybook/addon-actions';
 import { boolean } from '@storybook/addon-knobs';
-import { ifDefined } from 'lit/directives/if-defined.js';
 import { prefix } from '../../globals/settings';
 import textNullable from '../../../.storybook/knob-text-nullable';
 import './index';