Remove Growl support

Summary: We're about to get proper backends for Windows (D21661) and macOS (D22365). Judging by the code and comments this backend was never properly implemented anyway and will be dead code

Reviewers: Frameworks, broulik

Reviewed By: broulik

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22520