Drop Platform::screenSize()

It's effectively unused.

src/backends/x11/windowed/x11windowed_backend.h

@@ -86,7 +86,6 @@ class KWIN_EXPORT X11WindowedBackend : public Platform
     Q_OBJECT
     Q_INTERFACES(KWin::Platform)
     Q_PLUGIN_METADATA(IID "org.kde.kwin.Platform" FILE "x11.json")
-    Q_PROPERTY(QSize size READ screenSize NOTIFY sizeChanged)
 public:
     X11WindowedBackend(QObject *parent = nullptr);
     ~X11WindowedBackend() override;

src/platform.cpp

@@ -272,11 +272,6 @@ void Platform::setSceneEglDisplay(EGLDisplay display)
     m_eglDisplay = display;
 }
 
-QSize Platform::screenSize() const
-{
-    return QSize();
-}
-
 bool Platform::requiresCompositing() const
 {
     return true;

src/platform.h

@@ -106,13 +106,6 @@ public:
      */
     void setSceneEglGlobalShareContext(EGLContext context);
 
-    /**
-     * Implementing subclasses should provide a size in case the backend represents
-     * a basic screen and uses the BasicScreens.
-     *
-     * Base implementation returns an invalid size.
-     */
-    virtual QSize screenSize() const;
     /**
      * Implement this method to receive configuration change requests through KWayland's
      * OutputManagement interface.