Qt/GeneralWidget and GraphicsWidget: Remove unused member variable

m_xrr_config was used by the GeneralWidget::GetAvailableResolutions
function to get the list of supported fullscreen resolutions when
HAVE_XRANDR was set. aa4088a removed the ability to set that resolution
in the UI, leaving the GetAvailableResolutions function unused.

m_xrr_config is initialized in MainWindow which still uses it to toggle
fullscreen, but the references in GeneralWidget and GraphicsWidget
(which just ferried m_xrr_config from MainWindow to GeneralWidget) are
now unnecessary and removed in this commit.

Source/Core/DolphinQt/Config/Graphics/GeneralWidget.cpp

@@ -26,13 +26,10 @@
 #include "DolphinQt/QtUtils/ModalMessageBox.h"
 #include "DolphinQt/Settings.h"
 
-#include "UICommon/VideoUtils.h"
-
 #include "VideoCommon/VideoBackendBase.h"
 #include "VideoCommon/VideoConfig.h"
 
-GeneralWidget::GeneralWidget(X11Utils::XRRConfiguration* xrr_config, GraphicsWindow* parent)
-    : m_xrr_config(xrr_config)
+GeneralWidget::GeneralWidget(GraphicsWindow* parent)
 {
   CreateWidgets();
   LoadSettings();