Remove unused FindQt5PlatformSupport.cmake

Summary: This file was required for Qt 5.8. We require 5.11 as minimum version.

Test Plan: Grepped for the file being used in other cmake code, rebuilt KWin without it.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin, zzag

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16654