remove all platform conditionals not pertaining to linux or *bsd

Authored by sitter on Jan 30 2020, 12:13 PM.

Description

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