Remove completely unused color variables

`$color-cardinal` and `$gray-lightest` have no references in existing
SCSS

src/styles/variables.scss

@@ -10,14 +10,12 @@ $gray: #777 !default;
 $gray-dark: #585858;
 $gray-light: #969696 !default;
 $gray-lighter: #d3d3d3 !default;
-$gray-lightest: #f9f9f9 !default;
 
 $color-mine-shaft: #3a3a3a;
 $color-dove-gray: #626262;
 $color-gray: #818181;
 // Colors
 // ---------------------
-$color-cardinal: #bd1c2b;
 $brand-color: #bd1c2b !default;
 
 $button-text-color: $gray-dark !default;