[kded] Clean up notification handling

Summary:
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 a19794a3 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

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28691