move styling from js to css

IPython/html/static/notebook/js/menubar.js

@@ -52,7 +52,6 define([
 
     MenuBar.prototype.style = function () {
         var that = this;
-        this.element.addClass('border-box-sizing');
         this.element.find("li").click(function (event, ui) {
                 // The selected cell loses focus when the menu is entered, so we
                 // re-select it upon selection.

IPython/html/static/notebook/less/menubar.less

@@ -2,6 +2,7
   margin-top: 0px;
   margin-bottom: -19px;
   position: relative;
+  .border-box-sizing();
 
   .navbar {
     border-top: 1px;