Widget Customizer: Minor code cleanup.

* Remove type hinting from `WP_Customize_Widgets`
* Rename `inject_preview_css` to `print_preview_css`
* Remove stray `@todo`

fixes #27534.

git-svn-id: https://develop.svn.wordpress.org/trunk@27968 602fd350-edb4-49c9-b593-d223f7449a82