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