messageTray: Don't create notification policy on demand

This was changed in commit 8f15193b4 as a work-around for an ES6
class limitation, but now that Sources are GObject subclasses, we
no longer need to use that somewhat surprising pattern.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1131

js/ui/messageTray.js

@@ -762,12 +762,10 @@ var Source = GObject.registerClass({
 
         this.notifications = [];
 
-        this._policy = null;
+        this._policy = this._createPolicy();
     }
 
     get policy() {
-        if (!this._policy)
-            this._policy = this._createPolicy();
         return this._policy;
     }