pk: Clear the offline updates results after displaying them

Otherwise we get the notification every time that plasma starts, even if
there weren't any updates.

libdiscover/backends/PackageKitBackend/PackageKitNotifier.cpp

@@ -138,6 +138,8 @@ void PackageKitNotifier::checkOfflineUpdates()
 
         notification->sendEvent();
     }
+
+    PackageKit::Daemon::global()->offline()->clearResults();
 }
 
 void PackageKitNotifier::recheckSystemUpdateNeeded()