Do not call into NETWinInfo in AbstractClient::setSkipPager
There's a call to a virtual doSetSkipPager for performing this call and
Client implements it with the code which now is removed.
This fixes a crash when using setSkipPager for a Wayland window.
Crash which happens with a new test case (see dependent phab
request) is fixed.
Subscribers: plasma-devel, kwin
Differential Revision: https://phabricator.kde.org/D9865