Remove persistence from the kauth helper

Summary:
The way the helper works allows everyone with access to elevate to root
trivially, so persistence needs to be avoided.

Test Plan: .policy file has auth_admin now.

Reviewers: #plasma, davidedmundson, filipf, ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23078