Remove unused SCSS variables

What does this MR do and why?

Remove unused SCSS variables

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app/assets/stylesheets/framework/variables.scss

@@ -289,14 +289,11 @@ $dropdown-member-form-control-width: 163px;
 $filtered-search-term-shadow-color: rgba(0, 0, 0, 0.09);
 
 /*
-* Contextual Sidebar
+* Sidebar
 */
-$link-hover-background: rgba($gray-900, 0.06);
 $sidebar-toggle-height: 60px;
 $sidebar-toggle-width: 40px;
 $sidebar-milestone-toggle-bottom-margin: 10px;
-$sidebar-top-item-lr-margin: 8px;
-$sidebar-top-item-tb-margin: 1px;
 
 /*
 * Buttons