fix(checkbox): remove unsupported `is` prop and clean up Storybook

[ShubhamS2005]

Description

The CheckBox component no longer supports the is prop (was previously inherited from Box).
Storybook still had a control for is, causing TypeScript errors and Storybook crashes.

Steps to Reproduce

1. Open Storybook for Inputs → CheckBox

2. Try to use the is control in Storybook

3. Observe TypeScript error:
   type 'string' is not assignable to type 'ElementType | undefined'

4. Component still works, but Storybook shows errors and crashes when is is selected.

Expected Behavior

• No TypeScript errors
• Storybook loads cleanly
• CheckBox works normally

Actual Behavior

• TypeScript error when passing is
• Storybook crash or object showing instead of proper element

Proposed Solution

• Remove is control from Storybook for CheckBox
• Keep CheckBoxProps clean: Omit<BoxProps, 'is'>
• Ensure Checkbox functionality remains unchanged
• Storybook should run without errors

Notes

• Backward-compatible
• Tested manually in Storybook

[ShubhamS2005]

I have attached the following pr with it #1905