Bug 1039887 - Avoid unnecessary deletes in DOMRequestHelper.jsm. r=fabrice.

They're unnecessary because the entire object is later nulled out.

This in turn avoids creating 3,108 shapes when starting up B2G, which is about
73 KiB's worth.

dom/base/DOMRequestHelper.jsm

@@ -188,7 +188,6 @@ DOMRequestIpcHelper.prototype = {
       Object.keys(this._listeners).forEach((aName) => {
         this._listeners[aName].weakRef ? cpmm.removeWeakMessageListener(aName, this)
                                        : cpmm.removeMessageListener(aName, this);
-        delete this._listeners[aName];
       });
     }