Removed extraneous code line in auth guide code sample (#807)

Author: ioi0

docs/guide/synchronized-arrays.md

@@ -142,7 +142,6 @@ var app = angular.module("sampleApp", ["firebase"]);
 app.factory("chatMessages", ["$firebaseArray",
   function($firebaseArray) {
     // create a reference to the database where we will store our data
-    var randomRoomId = Math.round(Math.random() * 100000000);
     var ref = firebase.database().ref();
 
     return $firebaseArray(ref);