Drop unused mScreensInitialized variable in QWaylandDisplay

Unused since commit 78a70127

:
    Made wayland plugin single-threaded, yay :)

Change-Id: I1fae78d77179feaac79f5dba6dcad8084e3e97fa
Reviewed-by: Pier Luigi Fiorini <pierluigi.fiorini@gmail.com>
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>

src/client/qwaylanddisplay_p.h

@@ -202,7 +202,6 @@ private:
     QSocketNotifier *mReadNotifier;
     int mFd;
     int mWritableNotificationFd;
-    bool mScreensInitialized;
     QList<RegistryGlobal> mGlobals;
     int mCompositorVersion;
     uint32_t mLastInputSerial;