Remove support for KWin's QPA plugin for KWindowSystem

Summary:
KWin added a plugin for itself, thus it's no longer needed.

See: 02a0561016c22be906350bca68973f2818b727d7 in kwin.git

Reviewers: #kwin, #plasma, zzag

Reviewed By: #kwin, #plasma, zzag

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18564