Remove unused m_mutex variable in brcm-egl

Change-Id: I0c6b59eb9d59d6db4a71811562cdf740bfb0a06e
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>

src/hardwareintegration/client/brcm-egl/qwaylandbrcmeglwindow.h

@@ -83,8 +83,6 @@ private:
 
     int m_current;
     int m_count;
-
-    QMutex m_mutex;
 };
 
 }