Use prop() instead of removeAttr()

According to the jQuery upgrade guide:

> It is almost always a mistake to use .removeAttr("checked") on a DOM
> element. The only time it might be useful is if the DOM is later going
> to be serialized back to an HTML string. In all other cases,
> .prop("checked", false) should be used instead.

app/assets/javascripts/legislation_admin.js

@@ -8,11 +8,7 @@
           checkbox = $(this);
 
           checkbox.closest("fieldset").find("input[type='date']").each(function() {
-            if (checkbox.is(":checked")) {
-              $(this).removeAttr("disabled");
-            } else {
-              $(this).prop("disabled", true);
-            }
+            $(this).prop("disabled", !checkbox.is(":checked"));
           });
         }
       }).trigger("change");