Remove CenterPad and RightStick from Steam Controller (2015) internal state struct

These values are never used for the Steam Controller (2015), as it does not physically have these inputs.

src/joystick/hidapi/SDL_hidapi_steam.c

@@ -75,18 +75,10 @@ typedef struct SteamControllerStateInternal_t
     short sRightPadX;
     short sRightPadY;
 
-    // Center pad coordinates
-    short sCenterPadX;
-    short sCenterPadY;
-
     // Left analog stick coordinates
     short sLeftStickX;
     short sLeftStickY;
 
-    // Right analog stick coordinates
-    short sRightStickX;
-    short sRightStickY;
-
     unsigned short sTriggerL;
     unsigned short sTriggerR;