remove all platform conditionals not pertaining to linux or *bsd
Summary:
we do not support plasma on anything else, nor do we CI on other platforms,
so all these conditionals were unnecessary complexity and code nobody needs
Test Plan: still builds on linux
Reviewers: broulik, nicolasfella
Reviewed By: nicolasfella
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D27030