Drop Policykit backend

Summary:
PolicyKit was replaced by "Polkit-1" 8 years ago.

No supported Linux distribution is shipping security software that old.
We can safely drop it.

Test Plan: Compiles

Reviewers: lbeltrame

Reviewed By: lbeltrame

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27141