Remove widget arg from KNotification ctor

Summary: passing a widget will be removed in KF6 and doesn't really do anything here.

Test Plan: Builds

Reviewers: KDE Telepathy, akulichalexandr

Reviewed By: KDE Telepathy, akulichalexandr

Subscribers: broulik

Differential Revision: https://phabricator.kde.org/D26613