Remove underline header collapse links

IPython/html/static/tree/less/tree.less

@@ -192,6 +192,10 ul#new-menu {
                 padding-left: @dashboard_lr_pad;
                 padding-right: @dashboard_lr_pad;
                 line-height: @btn_mini_height;
+                
+                a:focus, a:hover {
+                    text-decoration: none;
+                }
             }
 
             .panel-body {