Don't remove border, make it transparent to prevent jumping of the page.

select2.css

@@ -189,7 +189,7 @@
 
 .select2-dropdown-open .select2-choice {
   border: 1px solid #aaa;
-  border-bottom: none;
+  border-bottom-color: transparent;
   -webkit-box-shadow: 0 1px 0 #fff inset;
   -moz-box-shadow   : 0 1px 0 #fff inset;
   -o-box-shadow     : 0 1px 0 #fff inset;