Remove _legacyUndefinedCheck from the change-list modules

Change-Id: I133851f71193434fb7380dbdc781c75573f89fd5
diff --git a/polygerrit-ui/app/elements/change-list/gr-user-header/gr-user-header.js b/polygerrit-ui/app/elements/change-list/gr-user-header/gr-user-header.js
index 93464b7..6942705 100644
--- a/polygerrit-ui/app/elements/change-list/gr-user-header/gr-user-header.js
+++ b/polygerrit-ui/app/elements/change-list/gr-user-header/gr-user-header.js
@@ -19,7 +19,7 @@
 
   Polymer({
     is: 'gr-user-header',
-    _legacyUndefinedCheck: true,
+
     properties: {
       /** @type {?String} */
       userId: {