Joystick: Remove the leftover separate slider handling

src/include/86box/gameport.h

@@ -36,7 +36,6 @@
 
 #define POV_X               0x80000000
 #define POV_Y               0x40000000
-#define SLIDER              0x20000000
 
 #define AXIS_NOT_PRESENT    -99999
 
@@ -50,7 +49,6 @@ typedef struct plat_joystick_t {
     int a[8];
     int b[32];
     int p[4];
-    int s[2];
 
     struct {
         char name[260];
@@ -67,15 +65,9 @@ typedef struct plat_joystick_t {
         int  id;
     } pov[4];
 
-    struct {
-        char name[260];
-        int  id;
-    } slider[2];
-
     int nr_axes;
     int nr_buttons;
     int nr_povs;
-    int nr_sliders;
 } plat_joystick_t;
 
 typedef struct joystick_t {