Remove $spacing-scale SCSS variables

Remove $spacing-scale SCSS variables from _variables.scss and replace usage with appropriate GitLab UI design token or variable.

Temp variables can also be removed:

/* Will be moved to @gitlab/ui by https://gitlab.com/gitlab-org/gitlab-ui/-/issues/1709 */
$gl-spacing-scale-48: 48 * $grid-size;
$gl-spacing-scale-75: 75 * $grid-size;
/* End gitlab-ui#1709 */

Edited May 28, 2024 by Scott de Jonge