calendar: Allow to dismiss resident notifications

Resident notifications are not really a thing anymore with the new
design, so all the user sees are some notifications that mysteriously
cannot be closed. That's utterly confusing, stop doing that.

https://bugzilla.gnome.org/show_bug.cgi?id=746860

js/ui/calendar.js

@@ -1252,10 +1252,6 @@ const NotificationMessage = new Lang.Class({
         this.setUseBodyMarkup(n.bannerBodyMarkup);
     },
 
-    canClose: function() {
-        return !this.notification.resident;
-    },
-
     _onClicked: function() {
         this.notification.activate();
     },