jQuery: Remove broken use of "removeAttr".

This function no longer sets properties to false, so the supported
way of doing this is to instead use prop(foo, false). Some tests
had to be fixed to accommodate this.

static/third/jquery-form/jquery.form.js

@@ -281,7 +281,7 @@ $.fn.ajaxSubmit = function(options) {
                 if ( useProp )
                     el.prop('disabled', false);
                 else
-                    el.removeAttr('disabled');
+                    el.prop('disabled', false);
             }
         }