Delete unused variables.

src/client/styles/scss/theme/_apply-colors.scss

@@ -268,13 +268,11 @@ pre:not(.hljs):not(.CodeMirror-line) {
 }
 
 .top-of-table-contents {
-  border-color: $bordercolor-toc;
-
   .grw-btn-top-of-table {
     fill: $color-link;
   }
   .grw-seen-user-list {
-    @include border-vertical('before', $bordercolor-toc, 70%);
+    @include border-vertical('before', $border-color, 70%);
 
     .btn {
       color: $color-seen-user;