Cleanup $commit SCSS variables

Cleanup $commit-* SCSS variables from _variables.scss and replace usage with appropriate GitLab UI design token or variable.

$commit-max-width-marker-color: rgba(0, 0, 0, 0);
$commit-message-text-area-bg: rgba(0, 0, 0, 0);

Do these need to be variables? Can we use transparent keyword instead?