InputCommon: Remove the Xlib backend

The XInput2 backend is more performant, so let's default to it and
remove the old, core-only backend.

Source/Core/InputCommon/ControllerInterface/ControllerInterface.h

@@ -22,9 +22,6 @@
 #endif
 #if defined(HAVE_X11) && HAVE_X11
 #define CIFACE_USE_XLIB
-#if defined(HAVE_X11_XINPUT2) && HAVE_X11_XINPUT2
-#define CIFACE_USE_X11_XINPUT2
-#endif
 #endif
 #if defined(__APPLE__)
 #define CIFACE_USE_OSX