Bug 1429111 - Remove unused nsITimer from element.Store. r=maja_zf

There is an unused nsITimer instance in element.Store#timer that
this patch safely removes.

MozReview-Commit-ID: APvPsXfHUk9

--HG--
extra : rebase_source : f936abcb20d42c0eb27cfdee62d461d276cc2e5c

testing/marionette/element.js

@@ -105,7 +105,6 @@ element.Strategy = {
 element.Store = class {
   constructor() {
     this.els = {};
-    this.timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
   }
 
   clear() {