Remove XInput2 XI_Motion support

Sadly, this interferes with the Steam overlay.

Fixes #304.

src/x11_platform.h

@@ -47,11 +47,6 @@
 // The Xinerama extension provides legacy monitor indices
 #include <X11/extensions/Xinerama.h>
 
-#if defined(_GLFW_HAS_XINPUT)
- // The XInput2 extension provides improved input events
- #include <X11/extensions/XInput2.h>
-#endif
-
 #if defined(_GLFW_HAS_XF86VM)
  // The Xf86VidMode extension provides fallback gamma control
  #include <X11/extensions/xf86vmode.h>
@@ -250,17 +245,6 @@ typedef struct _GLFWlibraryX11
         XGETXCBCONNECTION_T XGetXCBConnection;
     } x11xcb;
 
-#if defined(_GLFW_HAS_XINPUT)
-    struct {
-        GLFWbool    available;
-        int         majorOpcode;
-        int         eventBase;
-        int         errorBase;
-        int         major;
-        int         minor;
-    } xi;
-#endif /*_GLFW_HAS_XINPUT*/
-
 #if defined(_GLFW_HAS_XF86VM)
     struct {
         GLFWbool    available;