v1 Sass (LMS): remove duplicate color definition

The double definition of $blue-d1, $blue-d2, and $blue-d4 is
superfluous, so remove the second instance of each.

lms/static/sass/partials/base/_variables.scss

@@ -544,6 +544,3 @@ $darkGrey: rgb(136,145,161) !default;
 $extraDarkGrey: #3d4043 !default;
 $lightGrey1: $gray-l3 !default;
 $paleYellow: #fffcf1 !default;
-$blue-d1: shade($blue,20%) !default;
-$blue-d2: shade($blue,40%) !default;
-$blue-d4: shade($blue,80%) !default;