Revert "KWINDOWSYSTEM_DEPRECATED static void setMainWindow"

the function is only available from kwindowsystem 5.62, so more recent that our baseline.

This reverts commit 94e22f5a2fcc9f193ce0370805156ad5320d4141.

Change-Id: I6957a7d4438cd57c4a0eab29a9c615d45eaa2235
Reviewed-on: https://gerrit.libreoffice.org/79776
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
Tested-by: Julien Nabet <serval2412@yahoo.fr>
diff --git a/vcl/unx/gtk3_kde5/kde5_filepicker.cxx b/vcl/unx/gtk3_kde5/kde5_filepicker.cxx
index 0138a9c..a3c2ecf 100644
--- a/vcl/unx/gtk3_kde5/kde5_filepicker.cxx
+++ b/vcl/unx/gtk3_kde5/kde5_filepicker.cxx
@@ -268,8 +268,7 @@ bool KDE5FilePicker::eventFilter(QObject* o, QEvent* e)
        auto* w = static_cast<QWidget*>(o);
        if (!w->parentWidget() && w->isModal())
        {
            w->setAttribute(Qt::WA_NativeWindow, true);
            KWindowSystem::setMainWindow(w->windowHandle(), _winId);
            KWindowSystem::setMainWindow(w, _winId);
            if (auto* fileWidget = w->findChild<KFileWidget*>({}, Qt::FindDirectChildrenOnly))
            {
                fileWidget->setCustomWidget(_extraControls);