[qpa] Drop PlatformContextWayland which uses wayland egl for OpenGL

Summary:
Using the Wayland protocol for OpenGL is no longer used or useful. KWin
internal windows only use OpenGL through QtQuick. We either have the
sharing context (KWin uses OpenGL for compositing) or we have the
QPainter compositor which also turns QtQuick to use software renderer.

Thus a situation where the Wayland platform context is useful doesn't
exist any more. Removing it helps getting the QPA plugin Wayland free.

Test Plan:
Run nested KWin triggering Outline once with OpenGL and once
with QPainter compositor.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D19012