fix(css): remove whiteLight variable

A small MVC for fix #738 (closed)

In the heatmap.vue component we use a whiteLight variable which is the same as the white variable and is not used anywhere else in the @gitlab/ui and main GitLab project. This change aims to remove this extra variable and use white in its place.

This MR also removed all instances of white-light as a mixin and variable since it doesn't make sense to maintain it in the code base.

Screenshots