Drop deprecated code from Less files

less/navs.less

@@ -106,15 +106,6 @@
       border-color: @link-color;
     }
   }
-
-  // Nav dividers (deprecated with v3.0.1)
-  //
-  // This should have been removed in v3 with the dropping of `.nav-list`, but
-  // we missed it. We don't currently support this anywhere, but in the interest
-  // of maintaining backward compatibility in case you use it, it's deprecated.
-  .nav-divider {
-    .nav-divider();
-  }
 }