[client] Don't check resource version for PlasmaWindow

The checks for version are new in Wayland 1.10. We don't have that as
a dependency yet on build.kde.org and I don't want to needless raise
the requirement anyway.

The version check was only needed to support the theoretical use case
of a server having an older version. As the only user of PlasmaWindow
right now are also KWayland based it's nothing we need to check for.

So the check is just removed.