Remove the @todo note in css/components/buttons.css

Problem/Motivation

The css/components/buttons.css file contains the following lines.

/**
 * Buttons.
 *
 * 1. Enable z-index on buttons.
 * 2. Normalize 'line-height'; can’t be changed from 'normal' in Firefox 4+.
 * 3. Allow full range of styling in Webkit and Gecko.
 * 4. Use pixel units to ensure button text is centered vertically.
 * 5. Use rems to avoid the font size cascade of ems, with a pixel fallback for
 *    older browsers.
 * 6. Prevent fat text in WebKit.
 *
 * @todo Consider moving box-sizing into base.css under a universal selector.
 *   See https://www.drupal.org/node/2124251
 */

The linked issue (#2124251: Apply 'box-sizing: border-box' globally.) has been stale for 12 years and it is probably going to be closed.

Proposed resolution

Remove that @todo note from the css/components/buttons.css file.