Remove obsolete policy settings

Summary:
Those affect policy propagation and search path order for relative
includes in CMake code, none of which is needed here. This silences
a ton of warnings with CMake 3.10.

Reviewers: Build System, apol

Reviewed By: apol

Subscribers: Frameworks

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D10602