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.
Built from https://develop.svn.wordpress.org/trunk@27968


git-svn-id: http://core.svn.wordpress.org/trunk@27798 1a063a9b-81f0-0310-95a4-ce76da25c4cd