[kded] Clean up notification handling
Remove calls to deleteLater(), KNotification objects delete themselves after they timed out.
Remove unnecessary parent assignments.
Remove unnecessary id check before storing, this was introduced in a19794a3e as a supposed crash fix, but I doubt it actually fixes anything.
Test Plan: Spawned some connection (de)activated and error notifications
Reviewers: jgrulich, Plasma, broulik
Reviewed By: jgrulich
Differential Revision: https://phabricator.kde.org/D28691