[taskmanager] Simplifiy one of the two PID lookups

Summary:
KwindowInfo already has the PID, which we use in another case where PID is used.

More importantly I have a patch coming to KWindowSystem that returns the correct PID
for processes inside a PID namespace, but I don't alter the NETProperties.

Reviewers: #plasma, apol, hein

Reviewed By: #plasma, apol, hein

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22753

libtaskmanager/xwindowtasksmodel.cpp

@@ -513,7 +513,7 @@ QUrl XWindowTasksModel::Private::windowUrl(WId window)
     }
 
     return windowUrlFromMetadata(info->windowClassClass(),
-        NETWinInfo(QX11Info::connection(), window, QX11Info::appRootWindow(), NET::WMPid, NET::Properties2()).pid(),
+        info->pid(),
         rulesConfig, info->windowClassName());
 }