[wayland] Drop xdg-shell v5 support
Summary:
Drop xdg-shell v5 support since this protocol is obsolete and all popular
wayland compositors already did that.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D23573