move border-box-sizing from js to css

nothing to do in js, pure styling.

IPython/html/static/notebook/js/main.js

@@ -42,7 +42,6 b' require(['
     ) {
     "use strict";
 
-    $('#ipython-main-app').addClass('border-box-sizing');
     $('div#notebook_panel').addClass('border-box-sizing');
 
     var common_options = {

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

@@ -15,6 +15,10 b' body.notebook_app {'
     }
 }
 
+#ipython-main-app {
+    .border-box-sizing();
+}
+
 span#notebook_name {
     height: 1em;
     line-height: 1em;