Remove unused code in NotificationDbusInterface

Summary: The isRequest codepath is never used. The requestAnswer path is not implemented and after D9558 not needed anymore.

Test Plan: Sending notifications works like before

Reviewers: KDE Connect, albertvaka

Reviewed By: KDE Connect, albertvaka

Subscribers: albertvaka, KDE Connect

Differential Revision: https://phabricator.kde.org/D9559