Remove unused Stylus variables. (#2435)

Author: XhmikosR

layouts/css/_variables.styl

@@ -7,18 +7,12 @@ $bright-green = #028a00
 $light-green = lighten($node-green, 90%)
 $active-green = darken($bright-green, 20%)
 
-$yellow = #f7da03
-
-$blue = #3887be
-
 $light-gray3 = #f0f0f0
 $light-gray2 = #ccc
 $light-gray = #999
 $gray = #666
 $node-gray = #333
 
-$black = #000
-
 padded-link($padding)
   padding-left $padding
   padding-right $padding