platforms/x11: Drop XRandrScreens::displaySize()

The default implementation of Screens::displaySize() returns the
bounding rectangle of all available outputs.

In case the Xrandr extension is unavailable, there will be a fake
output whose dimensions are the same as the dimensions of all screens
combined.

plugins/platforms/x11/standalone/screens_xrandr.cpp

@@ -79,13 +79,4 @@ bool XRandRScreens::event(xcb_generic_event_t *event)
     return false;
 }
 
-QSize XRandRScreens::displaySize() const
-{
-    xcb_screen_t *screen = kwinApp()->x11DefaultScreen();
-    if (!screen) {
-        return Screens::size();
-    }
-    return QSize(screen->width_in_pixels, screen->height_in_pixels);
-}
-
 } // namespace

plugins/platforms/x11/standalone/screens_xrandr.h

@@ -24,8 +24,6 @@ public:
     ~XRandRScreens() override;
     void init() override;
 
-    QSize displaySize() const override;
-
     using QObject::event;
     bool event(xcb_generic_event_t *event) override;