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/MainWindow.cpp

@@ -1282,10 +1282,8 @@ void MainWindow::ShowGraphicsWindow()
               "display", windowHandle())),
           winId());
     }
-    m_graphics_window = new GraphicsWindow(m_xrr_config.get(), this);
-#else
-    m_graphics_window = new GraphicsWindow(nullptr, this);
 #endif
+    m_graphics_window = new GraphicsWindow(this);
     InstallHotkeyFilter(m_graphics_window);
   }