remove unnecessary imports, fixes #1912

Closes #1912

Merge request studip/studip!1256

resources/assets/stylesheets/scss/header.scss

-@import "./layouts.scss";
-
 /* --- header.css ----------------------------------------------------------- */
 #main-header {
     box-sizing: border-box;

resources/assets/stylesheets/scss/layouts.scss

 // TODO: SCSSify
 
-@import "./variables.scss";
-
 $page-margin: 15px;
 
 $content-width: 400px;

resources/assets/stylesheets/scss/variables.scss

@@ -44,7 +44,6 @@ $drag_and_drop_border: 1px solid $base-color;
     // The special handling for -- as #{"--"} seems to be neccessary to make
     // scss evaluate the color variables
     #{"--"}active-color: $active-color;
-    #{"--"}active-color: $active-color;
     #{"--"}activity-color: $activity-color;
     #{"--"}activity-color-20: $activity-color-20;
     #{"--"}activity-color-40: $activity-color-40;

resources/assets/stylesheets/studip.scss

@@ -46,6 +46,7 @@
 @import "scss/forum";
 @import "scss/gradebook";
 @import "scss/grid";
+@import "scss/layouts";
 @import "scss/header";
 @import "scss/headings";
 @import "scss/helpbar";
@@ -54,7 +55,6 @@
 @import "scss/index";
 @import "scss/fullscreen";
 @import "scss/globalsearch";
-@import "scss/layouts";
 @import "scss/links";
 @import "scss/lists";
 @import "scss/messages";