removed bg darken for navbar

Author: bsaraswat

notebook/static/notebook/less/variables.less

@@ -27,5 +27,4 @@
 @cell_border_width:             1px;
 @cell_left_border_width:        5px;
 
-@navbar-default-bg:               darken(#f8f8f8, 10%);
 @navbar-default-link-color:       darken(#777, 13%);