Remove all remnants of outdated notification system
The notification system is not used anywhere
anymore. There is no single emit of
newNotification within Elisa.
If new notifications are added to Elisa, they
should use a PassiveNotification from Kirigami.
More ui cleanup will follow later.
Reviewers: mgallien, ngraham
Reviewed By: mgallien, ngraham
Differential Revision: https://phabricator.kde.org/D23223