_sass: Remove custom colors

_sass/custom/variables.scss

 // Typography
 $body-font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
 
-// Colors
-$link-color: $blue-100;
-$sidebar-color: $body-background-color;
-
 // Layout
 $gutter-spacing: $sp-6;
 $gutter-spacing-sm: $sp-4;