Only use wl-shell for the ksmserver greeters on Qt < 5.12
Summary:
Apparently wl-shell fullscreen is broken on Qt 5.12 now, but we don't need this
workaround anymore.
BUG: 399918
Test Plan:
Clicked on logout, got the greeter. Without this patch, the greeter
was not visible.
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: davidedmundson, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D16381