Cleanup $label SCSS variables

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

$label-font-size: 12px;
$label-padding: 7px;
$label-gray-bg: #f8fafc;
$label-inverse-bg: #333;
$label-border-radius: 100px;

1. Remove static hex color values for suitable GitLab UI colors
2. See if we can remove variables in general