Removed combo-box explicit height

b/c it caused the carrot button to misalign with the combo box

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

@@ -98,7 +98,6 b''
 
         .widget-combo-btn {
             min-width: 138px; /* + 26px drop arrow btn = 164px */
-            height: 30px;
         }
 
         .widget-container {