css: remove the link-*-color variables

They have not been used since 92a2ad8 (Kill compass, 2015-03-24).

Besides, their names are slightly misleading, suggesting that they refer
to a color, when they actually have Boolean values.

Removing them will make the next step easier, where we want to bulk
convert the actual `*-color` variables from SCSS to CSS custom
properties.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

assets/sass/variables.scss

@@ -15,9 +15,6 @@ $light-font-color:   #9a9994;
 
 $link-color:         #0388a6;
 $link-hover-color:   lighten($link-color, 10%);
-$link-focus-color:   false;
-$link-active-color:  false;
-$link-visited-color: false;
 
 $base-font-family-fallback: Roboto Slab, DejaVu Serif, Georgia, Times New Roman, sans-serif;
 $base-font-family: Adelle, $base-font-family-fallback;