[wayland] Drop Qt extended surface

Summary:
This is only relevant on Qt and when WlShell is used.
This was only the case before Qt5.10.

We will depend on Qt5.12 next release.
Static Qt builds which are that old typically wouldn't contain QtWayland

Test Plan: Compiles. Not used in tests

Reviewers: #kwin, graesslin

Reviewed By: #kwin, graesslin

Subscribers: graesslin, zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18594