[wayland] Drop Qt extended surface
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
Differential Revision: https://phabricator.kde.org/D18594