Fix PageRowGlobalToolBarUI binding loop
We don't need to set the maximum width to 0 if it has no children because this happens implicitly anyway.
This fixes the following warning which appears in the console for all apps using an ActionToolBar:
file:///home/nate/kde/usr/lib64/qml/org/kde/kirigami.2/private/globaltoolbar/PageRowGlobalToolBarUI.qml:65:9: QML RowLayout: Binding loop detected for property "maximumWidth"
Tested in Discover, System Monitor and Kirigami Gallery's ActionToolbar page. No regressions found. Please test some more though.