Drop warning about unimplemented Screens::scale

Otherwise spams your log telling you to implement it.
We don't support per-screen dpi on X and never will, so this is a pointless warning.

Differential Revision: https://phabricator.kde.org/D9080

screens.cpp

@@ -108,7 +108,6 @@ float Screens::refreshRate(int screen) const
 qreal Screens::scale(int screen) const
 {
     Q_UNUSED(screen)
-    qCWarning(KWIN_CORE, "%s::scale(qreal screen) is a stub, please reimplement it!", metaObject()->className());
     return 1;
 }