Remove `closeButtonClassNames` prop from `Modal.Header`

[huyenltnguyen]

Description

Remove the closeButtonClassNames prop from Modal.Header once the Bootstrap overrides have been cleaned up in the main repo.

ui/src/modal/types.ts

Lines 15 to 24 in 284a135

	/**
	* /learn uses the `button` selector to override Bootstrap button styles,
	* and the selector also affects `ui-components`.
	* We need to forward the class specific to the close button from /learn
	* in order for the button to display properly.
	* Ref: https://github.com/freeCodeCamp/freeCodeCamp/blob/c1137b0900ad098fee2fca430d11f69dfc909d3b/client/src/components/layouts/global.css#L179-L192
	* NOTE: This prop is temporary and should be removed once we remove all CSS overrides from /learn.
	* Tracking issue: https://github.com/freeCodeCamp/freeCodeCamp/issues/52715
	*/
	closeButtonClassNames?: string;